I have exactly the same issue and can produce a repeatable case - please see my comment in the above forum reference (http://www.icefaces.org/JForum/posts/list/13151.page#56078).
As per my comment, I believe I have narrowed down the problem to the fact that GlassFish does not obey the servlet mapping :-
<url-pattern>/xmlhttp/* </url-pattern>
for the persistent faces servlet, due to the prescence of the extra space after the URL pattern.
My comment post lists the versions used. Basically I downloaded the current latest production versions of Eclipse Galileo, IceFaces and GlassFish together with all the latest plugins during the last couple of days, created a simple ICEFaces (nb "non-facelet 2.3, 2.4 or 2.5 GlassFish" version) dynamic web application in Eclipse, and hit the above error immediately when running the generated example page on GlassFish (works fine on Tomcat 6).
This is of particular concern as being a new user, it effectively presented a brick wall to using ICEFaces, as the tutorial samples all work but any generated apps of my own (e.g. a simple test with sortable table headers) failed to display correctly and work properly (see forum comment).
I spent significant time diagnosing the issue. Other developers new to ICEFaces and deploying to GlassFish may not be so patient and may end up dumping ICEFaces for something else because of this, which would be a shame as the fix is trivial (delete a single space).
It would be very helpful if this could at least be documented and ideally fixed by removing the space in the generated web.xml. I appreciate it is perhaps a grey area - I'm not sure what the spec rules are for url patterns re trailing spaces - in this case, it appears Tomcat 6 strips them and GlassFish does not, so this could either be a GlassFish bug or both a Tomcat and ICEFaces plugin bug.
It would also be helpful if there was a way of editing the template data for the plugin so that a correct web.xml is generated each time without needing to edit it - is this possible? I had a look but could not see how to do this.
ICEfaces project created by ICEfaces tooling "ICEfaces-1.8.1-Eclipse-3.4.2-plugins" works just fine on Glassfish 2.1.
The only thing that is different is that tutorial (Readme.pdf) is based on Tomcat. When you create ICEfaces project, you need to check "server supplied JSF implementation" as Glassfish already includes JSF impl. Libraries you need are
-ICEfaces library v1.8.1
-Apache Commons Library
-FastInfoSet (optional)
-JXL (optional)