To switch RubyMine to JDK 1.6 you should:

1. Open /Applications folder

2. Chose "Show package Contents" for RubyMine 1.1.1.app

3. Open "Contents/Info.plist" using "Property List Editor" or a  text editor

4. Change Root | Java | JVMVersion to 1.6*

5. In Root | Java | VMOptions change -Xmx512m to -Xmx700m because on MacOS Leopard java 1.6 is 64-bit and requires more memory than 32-bit java 1.5

6. Save changes to "Info.plist"

7. Delete idea file from "/Applications/RubyMine 1.1.1.app/Contents/MacOS"

8. then cp /System/Library/Frameworks/JavaVM.framework/Resources/MacOS/JavaApplicationStub64 to /Applications/RubyMine 1.1.1.app/Contents/MacOS/

9. rename the copied *JavaApplicationStub64* to *idea*.

10. add u+x permissions for *idea* file