I installed SQLExplorer 3.5.0RC6  in eclipse 3.4.1 (fresh install) Vista x64 and jdk 6u10x64: no errors but no perspective and the only plugin in plugin-details view was the help.
After many attempts, i tried with the standalone RCP client and got errors at startup.
Tried the same RCP client in my notebook (Windows XP and jdk6u10) and worked.
So uninstalled jdkx64 and installed the 32bit version in Vista, and RCP and plugins worked with no problem at all.
On the reasons why, I can say by issuing "java -version" the 64 bit jdk says "server hot spot" by default. I remember an article on javalobby or theserverside about jdk64 and OSGi or classloaders problems, but couldn't find.

Hope this helps.
Daniele