Carrier unlocked does not mean that it will work well with other US carriers, just that i can work with any SIM.
Point is you can take it anywhere in the world there is 3G coverage (at the right frequencies) stick a SIM in it and it will work. Not so with the iPhone (unless you bought an unlocked one, which is not possible in the US).
What I did find was the sale of $60-$100 devices/dongles that would enable you to use your iPad with T-Mobile, Verizon, etc.
Is it not only me that has trouble finding a clear statement about this (esp. on store.apple.com)?