The Apple iPad will come with two 3G data plans through AT&T. The first is just $30 a month with no contract. On this plan you can cancel at any time with no penalty, and use AT&T Wi-Fi hotspots for free. If that doesn’t sound good to you, maybe option number two is more to your liking.
The other plan is $15 for 250MB of data, also through AT&T. You can activate it directly from your iPad. That seems like a good deal, when you consider that a laptop plan will generally cost you $60 for unlimited data.