Around the World

Im currently on holiday in the US and Canada and was wondering if it would make sense to buy a cell phone to bring back to Europe (since electronics are generally quite a bit cheaper in the US).

Im mostly interested in phones similar to the iPhone in functionality, that is with good internet browsing functionality and a video/music player. As these phones are usually quite expensive, im thinking that the savings will be worth it.

So basically i would like to know if there are any major hindrances. For example:

1. Can you buy a phone unlocked without a subscription etc?

2. Will it be as easy as just putting my old SIM-card into the new phone?

3.
Can you buy phones in the US supporting UMTS
(or do you only get EDGE)?

4.
Is it possible to get an international warranty?

5.
Finally, where would be a good place to buy (type/name of store)? Im staying centrally in Seattle, Washington DC and NYC without a car.

Thanks for reading.