I have hit this on earlier versions, and each time I upgrade I forget the trick. So I'm posting it here so maybe I find it next time. And maybe any other Mac STS users who forget it can find it here too.
When you use an STS installer for Mac OS X the default installation location is not usually inside your /Applications. My STS 3.00 defaulted into a ~/springsource directory for example.
If you then try to use the "STS.app" within the sts-3.0.0-RELEASE folder to launch STS, you get an error:
If you move that ~/springsource directory off to /Applications/SpringSource and use the STS.app to launch, it works without error.An error occurred while trying to open "STS".
It's possible this occurred when I installed 2.8 on OS X 10.6 and it certainly happened installing 2.9 on OS X 10.7 and 3.00 on 10.7 as well.