I dropped by the Shinsaibashi Apple Store on Friday and they didn't have any external batteries available. I guess Apple still thinks the standard battery is good enough on its own.
Undeterred I visited BIC Camera in Namba tonight and found several external batteries that claimed to be iPod Touch compatible and thus should work on the iPhone 3G as well. Then I found the Sanyo battery section... WOW! They have some serious portable batteries these days!
Although it made no claims to be iPod or iPhone compatible I decided I would take a chance and buy one of these for about 5400yen:

Sanyo KBC-L2S
When connected to the iPhone 3G with a compatible dock cable it works perfectly!

It is a 5000mAh battery so even the iPhone 3G should be able to get through a day of heavy push email and 'net access.
The only downside is that it takes 7 hours to recharge with the included AC adapter or 14 hours to charge via USB. Not something that you can top up quickly unfortunately.
I'll post a followup after using the battery for awhile and let everyone know if it is as good as I hope.