I just did honey
So what you are telling me is:
1.) YOU downloaded iTunes 9
2.) YOU used your own iPhone 2G on your existing line (with a new full-sized SIM, obviously)
3.) YOU activated through iTunes 9 and obtained the unlimited data plan, without a hitch.
I'm not saying you're wrong/not being honest, I'm just trying to confirm that YOU did this (not read, heard, ect.) within the recent weeks and had success.