Stacey,
Verizon worked fine for us when we were on St John last year. At the time we used their "National One Rate" plan and we had no roaming charges. Others with different plans did pay roaming.
I know that ATT WIreless, Cingular and Spring PCS all have service in the USVI. There's no need to purchase a GSM sim card.
If you go to the British VI the story is very different. US cell phones won't work, best to buy a local prepaid landline phone card. Those are the best rates you will get in the BVI.

There are Cingular, Sprint PCS and ATT networks in the USVI. Verizon seems to work with no roaming for some folks and then with roaming and long distance for others. I read a message recently from one traveler who got mixed answers from the local verizon rep in the states - after speaking to several people no one could give a straight answer as to weather Verizon would work, and if it worked if it woudl be without extra charges. Another person, who has Cingular said their plan would not allow no-cost roaming in the VI although Cingular is a cell provider in the VI.

Best bet call your company and ask if your plan includes the USVI.

For reference, Cingular, Sprint, Centinial and ATT are local providers as is the local phone company.

--Islander

Well...I used Verizon and have the America's choice plan (I think that's what it is called, anyway).

In some places my 'roaming indicator' was lit and in others only my 'extended network indicator' was lit (extended network is not roaming). I only used the phone where it showed extended network and I wasn't charged any additional fees.

The connection wasn't the best though; each time I called there was about a one second or so lag between communications. Also, I was calling another cell phone in Pennsylvania each time I called, so maybe that has something to do with the lag?

All in all, I was satisfied and surprised that I got off for free with my Verizon phone in the USVI. (St Thomas).