I was worried about not being able to use Whispersync when attached to the "wrong" store. My plan is to eventually take the books I buy on Store A and (once liberated) send them as personal docs to Store B, otherwise keeping my device linked to Store B the rest of the time so I have Whispersync access to everything. We'll see how that works, though.
Oh, and I found one advantage to buying the Japanese 3G PW: price!
US: $199 converts to ¥16,370
Japan: ¥12,980 converts to $157.80
(Japan doesn't sell them with special offers, so that's the more-expensive US price)
So the worldwide 3G support effectively costs you an extra $41.20.
I think Amazon is pricing their new Japanese Kindles unusually cheap in a bid to jump-start the Japanese ebook market; even the WiFi PW is $22 cheaper than the same model in the US. And this in the market where the Sony PRS-T1 was literally twice the US price. Amazon is using its favorite tactic of undercutting the competition, methinks...