I carry a separate phone and PDA outside the US. If you have unlimited text messages inside the US INBOUND text messages are free outside the US on at least ATT. Outbound messages cost, but you can get around that (if you are on an unlimited BB plan) with a program like fishtext or by e-mailing to the person's cell phone (e.g. [email protected]).

I'm lucky because I still have unlimited domestic call forwarding on my mobile (Cingular Fast Forward -- grandfathered). I call forward my number to a roaming SIM via a US number. Look at something like Voicestick's next to nothing number.