The best plan may invoive two phones/SIMs:
1. A T-Mobile US postpaid international Blackberry plan can actually provide unlimited international data, even though it's not supposed to.
2. Any of the well known prepaid international SIMs can provide cheap voice service. Just pick the one that works best for you.

To add to DRN's comment - T-M USA has two BB models that can connect via UMA. That means you can make free calls to/from the U.S., and data connection would also be free. You would, of course, need access to wifi.
We have had occasions where we are charged for data on TM overseas (as we should) on the intl. BB plan.)
If you use TM to roam via UMA connection - set the BB to connect only via UMA so you don't, accidentally connect via cellular and incur roaming charges.
TM's UMA option is GREAT.

...mike

There might be a bit less wi-fi access, and even less of it free, than you hope.

Finding wifi is hit or miss. Lots of hotels now seem to offer it in many places we've been. And many coffee shops and restaurants. Of course if you are also visiting homes or offices, you'll usually get access there as well. I wouldn't count on this as a 24/7 option - but to use when you can - since it is free and the connection quality is usually very good. Works for us - altho since I am on an old T-Mobile plan where many countries are 30 cents/min. to roam, it isn't so critical for me.

...mike

If you have a device that supports it, I suggest Boingo Mobile. There are thousands of hot spots available all over the world for a flat ~$8/mo. The only issue is finding them. Boingo's website is actually not friendly to mobile devices and only give you an address, not a geographic position. If you've never looked for a street address in Japan, you have no idea how frustrating it can be.

I thought no one had unlocked the iPhone 3G yet, so if this is still true, you won't be able to use your iPhone.