That's a lot of "ons" for just a title but it's descriptive at least. I've been having some problems launching IDEA 8 on my macbook pro since upgrading to 8.0 final (the RCs were fine strangely enough). I tracked it down to a plugin that had been built against Java 6. Unbeknownst to me, IDEA 8 is hard coded to launch with Java 5. I have no idea (ha!) why but I know how to fix it. If you open up the package contents (right click, show package contents) and open the "Contents" folder you'll see a file called Info.plist. Edit this file with your favorite editor and change the line that says "1.5*" to say "1.6*" You'll also need to update the JVMArchs entry to list x86_64. Save the file (but don't close it in case you need to roll back this change) and start IDEA. When it's done starting up, click on the "Intellij IDEA" title in the menu bar and select "About InteliiJ IDEA" option. The window that pops up should now list your VM as some 1.6 variant.
Now, I'm sure that JetBrains had some good reason for tying to IDEA to 1.5. This change might break something down the line. I haven't used it enough to see if I hit anything like that but so far so good. Your milage may vary and, of course, I can't guarantee this won't trash your source files. But it seems to work so far. If you run into any issues, please leave a comment and tell me about it. I'm guessing if we run into anything it'll be some odd swing bug. Good luck with it.
A friend mentioned some menu rendering lag after trying this tip and fixed it by increasing his permgen space. Another pointed out the VMOptions IDEA plugin to help with editing those options. No more tracking down that file.