Got it to work by introducing both a delayed call and a generated event, just one of those isn't good enough.

The attached patch seems to fix the issue for me (patch was created using diff(1), not through mercurial).

I haven't tested this on anything else than IDLE 3.2 on OSX 10.6 with Tk 8.5.9 from ActiveState. 

If this gets committed I'd at the very least use this workaround only for Tk-Cocoa on OSX, and use the current code path on other platforms. That gives more code, but makes it clear than something special is being done for one specific platform.