NY Times travel section has a great article on this today (Sunday times - 9/28/03)

http://www.nytimes.com/2003/09/28/travel/28prac.html

Personally, I bought a tri-band phone here, and got a SIM card from Prague. You can buy a used phone over there for about $25, and then get a SIM card. I WOULD NOT RENT. It cost me $400 a few years ago, and it was not worth it.