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.