Probably everyone knows this but I just found it out this past September on a trip to Europe.
While I couldn't make calls on my phone, it was able to pick up the hotel's Wifi. Instead of booting up my computer I could check the weather, look up an address, check email, and even the TA forums on it. I never took it out of the hotel, but it was convenient to have internet access in my hand. Other people have mentioned that some, non-internet-dependant apps work on a smart phone.
So if you are coming to Key West from Europe, S. America, etc., and you are staying somewhere with internet access, you may be able to use your phone for everything except calls.