I've only been using wifi ever since I got my phone (I changed my settings from 3G to wifi only). I bought it unlocked and switched the network from Vodafone to Optus. I've just switched recently to the new Optus Prepaid Social plan, which gives me unlimited access to social networking, but now my internet isn't working at all.
So, I'm on AT&T, unlimited data. I'm planning on giving an iPad (wifi and cell) as a gift with an associated data plan to a person who's on Sprint and hates it (beside the point). I've been checking data plans and so far, no one comes close to just adding a line to my own plan with unlimited data at an additional $30/mo.
Apple's decision to pioneer the use of the new micro-sim format in the iPad has enabled operators, such as Optus and Telstra, to launch iPad specific mobile data plans even through they are not selling iPads. But if you can get a better deal on another plan, you can simply hack the standard sim.