You have to make sure your unlocked phone has the bands necessary for the carrier. If it is a quad-band phone, it should work with any GSM carrier in USA. If it is tri-band (900/1800/1900 or 850/1800/1900)it may work with one the other GSM carriers (T-Mobile or Cingular).
I hope you know that these GSM phones will not work with Sprint, Verizon or Nextel as they use CDMA or iDEN.
By buying an unlocked GSM cell phone you have the luxury of not having to sign long term contracts, but if you want a monthly plan with unlimited weekend minutes, etc. you may need to sign a 1 or 2 year contract.
You can also do the "pay as you go" plan.
I have recently returned to the states and I am shopping for a cellphone/plan. I have decided on the Motorola MPx200 because of its low SAR (.2). Unfortunately none of the carriers are selling this phone, so I must purchase it "unlocked" from a 3rd party vendor. I assume this means it has no programed connection to a carrier. Could someone please explain to me how to get my unlocked phone connected? Do I simply take it to a carrier's store and set up a plan, and ask them to configure the phone? Thanks in advance.
Aaron

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic