I had a couple of brief moments of discomfort while doing AlphaRev's NAND unlock (a.k.a. S-OFF, aka @secuflag) via Mac OS X / VirtualBox. While I'm sure the vast majority of people who'll do this have already done it I'll explain them quickly just in case it helps anyone.
I followed these instructions. (I'd post this there but apparently I'm too new to be trusted.)
Firstly, if you find that your handset is listed under the USB devices list but greyed out, make sure adb isn't connected (run adb disconnect).
Secondly, expect to have to re-establish the phone's virtual USB connection several times during the process -- just click the USB icon at the bottom of VirtualBox and select the phone. I spent a good few minutes waiting for progress when the flasher was actually waiting for me.
Aside: I did this to allow repartitioning of the NAND (if you're a CyanogenMod 6 / HTC Desire user you'll know just how little space is left on the /data partition of the already-tiny internal NAND for downloaded apps), but I don't really understand the locking mechanism. Clearly custom firmware could already be flashed to /system, but apparently /system was only writable while FASTBOOT was running, i.e. not while Android was running. Fine, but why can't you just repartition the NAND under FASTBOOT? Why does it require a patch to the recovery?