Error 21 Attempting to Jailbreak a locked 2G - anyone help?

Hello!

Here's bit of a puzzle:

\\ Mac OSX 10.5.6 (Automator-script-DFU re-enabled)
\\ iTunes 8.02
\\ AT&T "locked" 2G iPhone.
\\ Pwnage Tool 2.2.5

The iPhone will take any genuine Apple firmware restore I throw at it using iTunes (alt-click on Restore button, browse to firmware file 2.0.2, 2.1, 2.2 etc).

But the phone won't ever take a "custom" firmware file generated by Pwnage tool. iTunes eventually complains with an "Error 21" shortly after starting to apply the custom firmware (any version).

It's unlocking & activation I'm mostly interested in, at the moment after applying a stock Apple firmware iTunes tells me that the phone doesn't have a supported SIM card installed or, if I remove the SIM altogether that there's no SIM and I should slide one in.

Is there any way I can check for certain that the USB-DFU fix has been applied? Maybe that's the problem. Should I try booting into VMWare (Windows XP) and running Pwnage / iTunes from there? Presumably the OSX 10.5.6 DFU situation then goes away...

Thanks,

Steve