I developed to 2 custom VF Pages with associated custom tabs. The first VF page used a commandLink to navigate to the second VF with using a PageReference in a CustomController. Like so

<apex:commandLink action="{!SearchPage}" id="SearchLink">Do Search</apex:commandLink> <br/>

All good, the 2nd Page opened fine, EXCEPT that the Tab associated with the find page wasn't selected, instead the first tab was selected. Put more accurately the CSS wasn't updating to reflect the currently selected page. The target VF page in question isn't associated with an SObject (haven't tested this so I'm not sure if this is relevant). In this instance its a custom SearchPage

Ok what to do - I couldn't find any documentation or help, but I realised that the issue actually lay with the CSS... If I could manipulate the CSS I could fix the issue - step forward jQuery. I needed to change the CSS in the DOM and jQuery is just the tool.

**** WARNING***** this is a hack, if SFDC change the CSS or structure this code could break.

First setup you reference to the jQuery Lib in your target VF page

<script type="text/javascript"  src="{!URLFOR($Resource.jQuery, '/jquery-1.4.2.min.js')}"></script>

The tabs on the standard interface are wrapped in a table with the class name '.tab'

<table cellspacing="0" cellpadding="0" border="0" id="tabBar" class="tab"><tbody>
<tr>
<td nowrap="nowrap">
<div>
<a title="Home Tab" href="https://na7.salesforce.com/home/home.jsp">Home</a>
</div>
</td>

The td element of the currently selected tab will have a class added with the values like so:

<td nowrap="nowrap" class="currentTab primaryPalette">

So what we need to do is remove the class from the incorrectly highlighted tab and add it to the tab we want highlighted. Here is the jQuery to do this. (My jQuery skills aren't going to win any prizes, someone may have a shorter/better way of achieving this)

<script type="text/javascript">
j$ = jQuery.noConflict();
	
	j$(document).ready(function($) {
	
		var title = j$('.tab').find('.currentTab > div > a').attr('title');
		
		if (title != 'Search Tab - Selected') {
			j$('.tab').find('.currentTab').removeClass('currentTab primaryPalette');
			
			var x= j$('a[title|=Search Tab]').attr('title', 'Search Tab - Selected');
			
			j$(x).parent().parent().addClass('currentTab primaryPalette');
		}
	});

</script>

 The title of the a link of the seleted tab is always myTabName Tab, if its currently selected is myTabName Tab -Selected. The code basically removes the class from the incorrect tab and adds it to the one we want.

The double use of .parent() (please feel free to offer a better technique) navigates from the <a> through the <div> back the current <td>, this is where the class needs to be added. And it works!!! But its a hack