Apple recently released the "Java for Mac OS X 10.6 Update 5" (together with the corresponding update for 10.5). This could cause the 64bit versions of STS to fail at startup. You will get a message like:
This happens due to a deleted link in the Java installation after the update. You can solve this by opening the content of STS.app/Contents and manually editing the "Info.plist" file. There you find a line (close to the bottom):
A Java Runtime Environment (JRE) or Java Development Kit (JDK) must be available in order to run STS.
No Java virtual machine was found after searching the following locations:...
If you are running Mac OS X 10.6, you can just delete this line from the file (or put it in HTML comments). For 10.5, you can replace "1.6.0" in the path with "1.6", that should do it. Save the file and you are done.
But again, this is only necessary for the 64bit versions of STS, not the 32bit ones.