Only need data occasionally? Verizon offers their $10 per day no contract service for EVDO on the go via embedded or add-on wireless modem card/USB stick and now Virgin Mobile the no-contract kings, are offering EVDO Rev. A data for notebooks and computers with no commitment. The service works with their Novatel Wireless MC760 USB wireless modem, sold only at Best Buy Mobile stores.
Rates run as follows:
$10 for 10 days and 100 megs of data $20 for 250 megs and 30 days $40 for 600 megs and 30 days $60 for 30 days and 1 gig of data
Overall, that's not a lot of data-- heavy users be warned. Just surfing and doing email? Then you should do fine. But stay away from iTunes movie downloads and P2P. 1 iTunes HD movie eats up 500 megs! You get 5 gigs per month for $60 with the contract carriers, but then they do require a contract.