I realize you said cell phone (and cheaper), but just maybe the Vonage WiFi Phone could fit your needs if you're going to be near a network (and you're also looking for an excuse to try out VOIP).

It's a wireless internet phone that uses Vonage over WiFi hotspots (open or secure). You could even take your own access point to connect to an existing network. (Even when you're not near a network, you have built-in voicemail.

Costs about $100 at set-up (talking them out of the activation fee), then the monthly plan, with either 500 U.S/Canada minutes or else unlimited minutes. I don't yet have one, but it's in the mail as I type this. (I already use Vonage in Germany over their router setup.)