This week's book giveaway is in the OCPJP forum. We're giving away four copies of OCA/OCP Java SE 7 Programmer I & II Study Guide and have Kathy Sierra & Bert Bates on-line! See this thread for details.
I see that this one was asked a while ago, but I ran into the same problem recently. So, just to help others, I found that the problem for me was the name of the variable that I was storing the xslt result in was bad. It contained a '-' which was not a problem for jstl, in general, but the xml jstl tag library doesn't like it.