I installed Snow Leopard the other day in under 60 minutes without a hitch.... or so I thought.
When trying to run my App Engine project with GWT, I received the following fatal error:
You must use a Java 1.5 runtime to use GWT Hosted Mode on Mac OS X.
I did a search and quickly found the culprit. Snow Leopard installs with the 64bit version of the Java 6 JVM but GWT only runs with 32bit version of Java 5 JVM (as of now).
The good news is that until Google fixes the issue you can downgrade to the 32bit version of the Java 5 JVM. Here are the instructions. Good luck!