Perhaps you already know this, but just in case, I think you have to make sure that in STS.ini you put the "-vm" and "/usr/local/openjdk7/bin/java" on two separate consecutive lines.
The Eclipse thingy that is reading the .ini file is *very* picky about where the line breaks are. I recall having problems with that myself a while back.
Perhaps this is not your problem... but just in case it, I though I'd mention it. If you already have the arguments on separate lines, be so kind to ignore my post and pretend I didn't post anything :-)
Kris De Volder -- SpringSource