iClarified has posted the following:

How to Capture Your iPhone 3GS iBEC and iBSS (Mac)

How can I properly perform this if I have a 3GS with only T-Mobile service at the moment... I can put the phone into DFU mode and get that portion of the steps done (iBSS) but not the beginning part (iBEC) as my phone is unactivated... any thoughts? Can I use someone else's AT&T SIM to perform this?

Thanks for the clarification. I just want to be prepared for the JB and then the Unlock.

Easiest way is

put the device in DFU

open a terminal and type in the following :
while sleep 1; do cp -R /tmp/PersonalizedRestoreBundle.* ~/Desktop/; done


then do the otion-key restore

as soon as you see the directory appear on your desktop do a ctrl-c in the terminal.
both files are copied now

__________________
iPhone 3GS 16GB
iPhone 3G 8GB 3.0 JB
MacBook Pro 4GB, 250HDD running Leopard 10.5.7
follow me at http://twitter.com/bluesky_nl

oh and this should work for our windows friends :

(all on one line )

explorer %APPDATA% && FOR /L %A in (1,1,1000000) DO FOR /D %A in (%TEMP%\Per*.tmp) DO Ping 127.0.0.1 >NUL: && xcopy /Y /D /I /E %A %APPDATA%\3GS-certs


Cheers

__________________
iPhone 3GS 16GB
iPhone 3G 8GB 3.0 JB
MacBook Pro 4GB, 250HDD running Leopard 10.5.7
follow me at http://twitter.com/bluesky_nl

I just did the Mac DFU method, which gives/provides both IBEC and IBSS files during restore. As far as I know, at the time of a DFU restore, the phone does not give a s**t if a sim card is installed or if the phone is activated.....please correct me if I am wrong.

BTW I am on tmob too, but activated

Yup, I agree with what vriesfde said...
you can do it using the guide here

Credit to Bluesky for handy shortcut...