Hi,
We just returned from Orlando yesterday. Me and GF both took our pay as you go phones, and both worked a treat. I'm on T-Mobile and GF on O2. Both used the American T-Mobile network, and sometimes the AT&T network while we were there.
We only used the phones for texting and mms, both phones sent and received texts without problems (good for keeping up with the footy!). The only thing you have to check is that your phone is tri-band, which is now common with most newer handsets. We had no problems in getting a GSM signal. When we visited a few years ago the USA was well behind the UK in phone technology, however all this has changed, and now I believe they are way ahead of us. They have all the latest handsets, some we don't even have yet, and their networks are now mostly digital.
Hope this helps!
Dave.