I dropped a cut down T-Mobile PAYG sim in my iPhone 4 after a friend was talking about how much faster his speeds became contract became after he switched from PAYG. Do they limit the speed of the PAYT Sim's i get like 190kbps and 55kbps (which is much slower than he got) See the pic below..
http://yfrog.com/mrphotokp
http://yfrog.com/mrphotokp