Skodisana - don't give up hope just yet ;)  Campaign Member, though it is a standard object, is the ONLY exception to the rule.

You can create public sites pages that directly edit/update the Campaign Member object.  Combined with campaign member custom fields (to be released mid July), this is a perfect combo for event registration pages. 

But don't take my word for it - check out my live example:

http://jdk470.force.com/?id=00v3000000JIrl6AAD

A couple more ID's you can change:

• 00v3000000Jp9Ep 
• 00v3000000KsPdo

Note I haven't yet included filters to determine the allowable status values for a campaign as most people will want to create Status as a hidden field & define a default value.  The real power here is with Campaign Member custom fields.

Mid July, look for Campaign member custom fields, which make this a lot more interesting.

Tips when setting up your Campaign Member sites pages:

• Need Read on Campaign, and Read on Leads (if a lead) or Read on Contacts (if a Contact campaign member)
• Sharing cannot be private, or the campaigns & leads/contacts must be explicitly shared with the  Guest User License profile

PS - there's a good chance I'll change / break the above link at some point in the future, but rest assured this works. 

Some code to get you started:

Apex Controller:

public class UpdateCampaignMemberClass{

public final CampaignMember cm;

private ApexPages.StandardController cmController;

public UpdateCampaignMemberClass(ApexPages.StandardController controller) {
cmController = controller;
this.cm= (CampaignMember)controller.getRecord();
}

public PageReference updatecm() {
try{
update cm;
}
catch(DmlException ex){
ApexPages.addMessages(ex);
}
Id = :cmid limit 1];

return new PageReference('');
}
public string getStatus(string Status){

where Id = :cmid limit 1];
return 'Status';
}

}

 VisualForce Page:

<apex:page standardController="CampaignMember" extensions="UpdateCampaignMemberClass" sidebar="false" showHeader="false">

<script type="text/javascript">

function getQueryVariable(variable) {
var query = window.location.search.substring(1);
var vars = query.split("&");
for (var i=0;i<vars.length;i++) {
var pair = vars[i].split("=");
if (pair[0] == variable) {
return pair[1];
}
} }

function showAlert(variable){
alert(variable);
}
</script>

<table align="center" bgcolor="#ffffff" border="0" cellpadding="0" cellspacing="0" width="800">
<tbody>
<tr>
<td align="left" height="60" valign="top">
<p>
<!--<apex:image id="CloudForceTop" value="{!$Resource.CloudforcebannerGIF}" width="800" height="150"/>
-->
</p>
<table align="center" border="0" cellpadding="0" cellspacing="0" width="760">
<tbody>
<tr>
<td colspan="2" height="60"><strong><font color="#d76d43" face="Arial, Helvetica, sans-serif" size="5">Sign up for our Customer Conference! </font></strong></td>
</tr>
<tr>
<td height="130" valign="top" width="450"><font face="Arial, Helvetica, sans-serif" size="2"><strong><font size="3">Learn best practices from customers like yourself.</font></strong><br/>
At this conference you'll meet like minded collegues with similar
issues. We'll have a host of sessions catered to your individual needs.
This will be the biggest and best event we've had yet and expect top
thought leaders such as Bill Gates and Warren Buffet discuss the state
of the economy and how you can mitigate risk.</font></td>
<td rowspan="2" align="center" valign="top"> <!--<apex:image id="EventCustomoreQuote" value="{!$Resource.EventCustomerQuote}" width="185" height="133"/>
<apex:image id="EventCustomerQuote2" value="{!$Resource.EventCustomerQuote2}" width="185" height="155"/>
-->
</td>
</tr>
<tr>
<td align="center" valign="top">
<apex:form >

<table border="0" width="500">
<tbody>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="CMID" style="font-size: 14px;">Campaign Member ID</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<td><apex:Outputfield value="{!campaignmember.Id}"/></td>
</tr>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="first_name" style="font-size: 14px;">First Name</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<td><apex:InputField value="{!campaignmember.Lead.FirstName}"/><apex:InputField value="{!campaignmember.Contact.FirstName}"/></td>
</tr>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="Last_name" style="font-size: 14px;">Last Name</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<td><apex:InputField value="{!campaignmember.Lead.LastName}"/><apex:InputField value="{!campaignmember.Contact.LastName}"/></td>
</tr>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="Status" style="font-size: 14px;">Status</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<td><apex:inputfield value="{!campaignmember.Status}"/></td>
</tr>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="Hotel" style="font-size: 14px;">Hotel</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<!-- <td><apex:inputfield value="{!campaignmember.Hotel__c}"/></td>
</tr>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="MealPreference" style="font-size: 14px;">Meal Preference</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<td><apex:inputfield value="{!campaignmember.Meal_Preference__c}"/></td>
</tr>
<tr>
<td>
<div align="right"><strong><font face="Arial, Helvetica, sans-serif" size="2"><label for="Registered" style="font-size: 14px;">Registered</label>
</font></strong></div>
</td>
<td>&nbsp;</td>
<td><apex:inputfield value="{!campaignmember.Registered__c}"/></td>
-->
</tr>
<tr>
<td>
</td>
<td>&nbsp;</td>
<td><apex:commandButton value="Register Now!" action="{!updatecm}" /></td>
</tr>
</tbody>
</table>

</apex:form>

</td>
</tr>
</tbody>
</table>

<!-- <apex:image id="CloudForceBottom" value="{!$Resource.CloudforcebottomGIF}" width="800" height="250"/>
-->
</td>
</tr>
</tbody>
</table>
</apex:page>

 The page looks a bit messy with all the tables for alignment, but does the trick.  Note I've commented out the pictures & a few other fields.