thanks. I've of course looked into it with MallocDebug, ObjectAlloc 
and friends. I have been looking for the wrong kind of object though: 
I had looked for Core Graphics objects that are used to cache the 
graphics rather than for the objects that manage the menu items. And 
although their release/retain/autorelease calls are balanced, it 
seems that - for whatever reason - Cocoa decides to not flush the 
autorelease pool too often, only sometimes or not at all. Adding 
another NSAutoreleasePool on top and around the main entry point for 
event handling in RB fixed the problem.

So, this issue will be fixed in preview 6.

Cheers, 
Felix