iPad 3G data plan

Hi Gary, I want an iPad but am mostly going to use it at home and work, but I also once in a while do some traveling and would like to use it on the road. Is there a data plan that isn’t monthly, since I could go a couple of months where I would only use the wifi and then want to use the 3G network? I’d rather not pay $15/month when there will be stretches of a time when it is not needed. Any advice?
—–
Kevin E Durr