Check this link.
As you can see from there, all you need is to select the apropiate cellphone no matter what company. As soon as you have a cellphone with data capability (intenal modem) you are all set.
In my case I use AT&T basic plan + and Ericsson R289LX. The speed for browsing Internet is not so good but for emails is great. If you buy a cellphone with GPRS capabilities the seed would be nice.