the micro sim is just a smaller shaved down version of a regular sim.
If you shaved down the sides of a regular t-mobile sim card you can get it to fit, but you will be limited to only edge speeds in the US
I don't think anybody could try it yet since there are no 3G's out there in the wild.
So, the idea would be to take an iPhone SIM, cut it down to micro-SIM size to use in the iPad, then be able to put the micro-SIM in the adapter so that you could use it back in the iPhone? Would that work? The iPad would just automatically sift out the data portion so that the AT&T network would think it was actually a valid iPhone? Hmmm...I'm not sure about that. AT&T tracks the IMEI number of the device and compares it to the plan- not sure they'd go for that.
If we're talking buying a T-Mobile data plan, cutting down the SIM to use in an iPad, it would be EDGE-only, and would be $50/month, and 2 year contract. You'd have to REALLY want to use T-Mobile (or have to)...