This is just a repeat of what I said in the official heimdall thread
I just tried using it with the SGS2. Nothing radical, since I have zero experience with heimdall and Galaxy phones.
1) I put the phone into download mode, following instructions I got here.
2) I connected the phone to my fedora 11 install (yes, I'm way behind the times!)
3) I ran lsusb and got:
Bus 001 Device 002: ID 04e8:685d Samsung Electronics Co., Ltd
It doesn't mention the words "mobile phone" as I think it does for the S1, but I may be wrong about that.
4) I ran:
heimdall close-pc-screen --verbose
according to instructions earlier in this thread (page 26).
5) I got:
Failed to detect compatible device