Is it possible to root the Samsung Galaxy S II without installing a custom kernel or otherwise flash custom firmware?
Why do you want to avoid flashing Chainfire's kernel? From XDA:
Rooting voids your warranty just the same as flashing does, and Odin is virtually guaranteed to save you if you ignore the instructions while flashing and screw something up.
You can also re-flash the stock kernel afterwards, which will remove CWM while still leaving you rooted.
Edit: Gingerbreak apparently only worked on test devices.