I do indeed plan on calling AT&T (oh how I love spending my time dealing with them), and adding international roaming on my line of service before I head out, thus I'll be using my own SIM. I understand it may get pricey, and it will be slightly annoying having to dial international out to Japan while IN Japan. But to alleviate some of the monetary pain, I will be turning the data off completely (there is an option on the Bold for turning data services "Off when roaming"). We'll see if I can reconcile the calls as firm charges when I get back...
What are the price ranges on renting cell phones? I guess if they're not that bad I could rent one, but I don't see me spending a whole lot of time on the phone, so I don't think using international roaming on el BB would be too terrible.