Not really.
At least here in the US, iPhone will not work with Sprint or Verizon
It would if they supported GSM
and only AT&T and T-Mobile can use the device on theirs networks,
technically yes. legally only ATT is supposed to be servicing the phone. but we aren't going to start that debate
but only AT&T has visual voicemail.
did you ever stop to consider that that could be easy to fix with a little nugget of software on the carriers side. software that Apple only gave ATT since they are the only ones that are supposed to be handling the phone. but once the contract is up they might give that little nugget to all the companies currently supporting GSM along with another one that would remove the SIM lock for older phone owners.
who knows at this point. according to the talk ATT has at least another year so in about 12-18 months we could have our answer.