Several people have mentioned about the Tomcat extension not working with JDK 5.0 (1.5.0). I had the same problem myself, but I found a way around it. However, you'll need plenty of hard drive space and bandwidth to kill (I, for one, used my school's bandwidth. Not the best, but faster than a modem!).
It seems that the Tomcat installer checks for something that doesn't exist in the JDK 1.5.0. What this is, I don't know; a file, a registry entry, something. So, I figured I'd try something: Installing JDK 1.4.2 *and* JDK 1.5.0 (actually, I think I have 1.5.1 or 1.5.0-1 or something...it's the second release of Java 5). It worked! 1.4 installed without a problem, and didn't seem to interfere with 1.5's doing - 1.5 seems to still run everything itself. So, this gives you the 1.4.2 thing the installer needs, and still the functionality of Java 5.
I haven't tested this too extensively, including uninstalling 1.4.2 afterwards and seeing which one Tomcat actually uses once it's installed (Does anyone know how to check this?) - all I know is that when I unload Tomcat with the bat script, it uses the 1.5.0 directory as JAVAHOME.
So, to make a long story short: If you want 1.5.0 functionality and 1.4.2's variables that let XAMPP's Tomcat Installer run, just install 1.4.2 alongside 1.5.0.
If anyone else tests this more extensively, let me know. I'm fine with how it is right now, since I've got plenty of room on my hard drive...however, ideally we can find out *what* the installer accesses, so we can add it rather than downloading the whole of Java. After all, why download and install over 50 megs when all you need is a registry key?