I'm pretty sure I'm just trying to do some straightforward AJAX here.  When I use oncomplete or reRender widhin actionFunction, I see the following Javascript error (via Firebug):

element.selectNodes is not a function

And the Javascript engine basically choke at this point.   The error happens within the Salesforce AJAX library: 

3_3_0.GAorg.ajax4jsf.javascript.AjaxScript on line 101.  This occurs at a point where the code does a try{selectNodes()} to figure out of it should run either selectNodes() or getElementsByTagName().  If the try{} fails, the catch(){} should run the getElementsByTagName().

I've seen this in FF3.5.6, though things seem to work fine in FF3.0.16.   In IE7 and IE8 I get data back inconsistently.  Generally if I put trash data in yahooProduct and then try good data, I will things will work fine.  However, until I do this special sequence, the AJAX will not work in IE.  I know, this part this sounds like a bug of my own making... probably is... but for the life of me I cannot pin point it.  The code isn't all that complicated.

If I take out oncomplete and reRender, I get no Javascript errors.  Of course, I have no way to see if the AJAX did anything, either.

Strangely, I can find no references to this issue on the discussion boards.  I'm not doing anything particularly fancy.  Anything stand out in my code below?

My Visualforce:

<apex:page standardController="Purchase__c" extensions="PurchaseExtension" recordSetVar="purchases" title="New Purchase">

<apex:sectionHeader title="Purchase Edit" subtitle="New Purchase"/>

<apex:messages layout="table"/>

<style>
td.labelCol {
vertical-align: middle;
}
</style>
<apex:form styleclass="p-form" id="p_form">

<apex:pageBlock title="Purchase Edit" mode="edit">

<apex:actionFunction name="updateProductDetails" action="{!updateProductDetails}" immediate="true" reRender="field_table">
<apex:param name="firstParam" assignTo="{!yahooProduct}" value="" />
</apex:actionFunction>

<apex:outputPanel id="field_table">
<table cellspacing="0" cellpadding="0" border="0">
<tbody>
<tr>
<td class="labelCol requiredInput">Contact</td>
<td class="dataCol col02" style="height:20px;">
<apex:inputField value="{!p.Contact__c}" required="true"/>
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Purchase Date</td>
<td class="dataCol col02">
<apex:inputField value="{!p.Purchase_Date__c}" required="true"/>
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Yahoo! Product</td>
<td class="dataCol col02">
<apex:inputField value="{!p.Yahoo_Product__c}" required="true" onchange="updateProductDetails(this.value)" />
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Item Description</td>
<td class="dataCol col02">
<input disabled="true" id="item-desc" type="text" value="{!productDesc}"/>
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Item Price</td>
<td class="dataCol col02">
<input disabled="true" id="item-price" type="text" value="{!productPrice}"/>
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Quantity</td>
<td class="dataCol col02">
<apex:inputField value="{!p.Quantity__c}"/>
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Total</td>
<td class="dataCol col02">
<input id="total" type="text"/>
</td>
</tr>
<tr>
<td class="labelCol requiredInput">Ship Date</td>
<td class="dataCol col02">
<apex:inputField value="{!p.Ship_Date__c}" />
</td>
</tr>
</tbody>
</table>
</apex:outputPanel>

<apex:inputHidden value="{!p.Name}"/>
<apex:inputHidden value="{!p.Contact__c}"/>

<apex:commandButton value="Save" action="{!save}"/>
<apex:commandButton value="Cancel" action="{!cancel}"/>

</apex:pageBlock>

</apex:form>

</apex:page>

 and my Extension:

public class PurchaseExtension {
public Purchase__c p {
get {
if (p == null)
p = new Purchase__c();
return p;
}
set;
}

public Contact c {
get {
String contactId = Apexpages.currentPage().getParameters().get('id');
if (c == null) {
c = [select c.FirstName,c.LastName from Contact c where c.id = :contactId];
}
return c;
}
set;
}


String productDesc, productPrice;
public String getProductDesc() {
return productDesc;
}
public String getProductPrice() {
return productPrice;
}

String yahooProduct = 'init';
public void setYahooProduct(String p) {
yahooProduct = p;
}
public String getYahooProduct() {
return yahooProduct;
}

public PageReference updateProductDetails() {
if (yahooProduct != null) {
List<Yahoo_Products__c> yp = [select p.Item_Description__c,p.Price__c from Yahoo_Products__c p where p.Name = :yahooProduct];
if (yp.size() > 0) {
productDesc = yp[0].Item_Description__c;
productPrice = ''+yp[0].Price__c;
} else {
productDesc = 'No product details avalable';
productPrice = 'No product details avalable';
}
} else {
productDesc = 'No product?';
productPrice = 'No product?';
}
return null;
}

public PurchaseExtension(ApexPages.StandardSetController ctlr) {
p.Contact__c = c.Id;
p.Purchase_Date__c = Date.today();
}

public PageReference save() {
PageReference pr = null;
// stub
return pr;
}

public PageReference cancel() {
// stub
return null;
}

public static testMethod void testSave() {
Test.setCurrentPage(new PageReference('/?id=003A0000004JoTl'));
List<Purchase__c> pList = new List<Purchase__c>();
ApexPages.StandardSetController ctlr = new ApexPages.StandardSetController(pList);
PurchaseExtension e = new PurchaseExtension(ctlr);
e.c = new Contact(FirstName = 'Foo', LastName = 'Bar');
e.p.Yahoo_Product__c = 'a04A000000101DT';
e.save();
System.assertNotEquals(e, null);
}

public static testMethod void testUpdateProduct() {
Test.setCurrentPage(new PageReference('/?id=003A0000004JoTl'));
List<Purchase__c> pList = new List<Purchase__c>();
ApexPages.StandardSetController ctlr = new ApexPages.StandardSetController(pList);
PurchaseExtension e = new PurchaseExtension(ctlr);
e.setYahooProduct('HistProd');
e.updateProductDetails();
String newDesc = e.getProductDesc();
System.assertNotEquals(newDesc, null);
}
}