Yah, here is what this guy I work with had to say about it in an e-mail:
"Yes you can. However as with everything else technology-wise in this world there are some caveats.
First of all you need to know the kind of network you are going to be using the phone with. There are two major types in the US. There is CDMA/TDMA. this network does *not* use a sim card. For this net you get a phone that does not use the sim card and you can unlock it with softward readily found on the net. Typically you will need some kind of data cable to attach to the computer. Then you need to program the phone with all the relevant network configuration stuff. This you will need to get from your provider.
The other type of network is GSM. This is the one that uses sim cards. You will still need to unlock the phone if it is locked by another company. Basically it's the same thing. You configure it and then put your sim card in and it should work.
YMMV and it is fairly easy to brickify a phone this way. It works and I have done it. But there are on guarantees. '
