All of you frequent international travelers will no doubt eye this item with interest: USB Fever has an extended iPhone 3G / 3Gs battery which has a dual SIM adapter. Many people who travel regularly to the same international destination will keep a SIM for each country, which helps them save on data and voice charges; being able to keep both SIMs in the same device means easy swapping between the two.  The added benefit of having an external 1900 mAh battery means that your iPhone charge might last through several travel days.

From the USB Fever site:

Features:
- One iPhone (3G / 3G s) with 2 SIM Cards (see note below)
- No need to cut your SIMCARD
- STK Menu
- Unique design
- Portable power pack (Enough Power to hold your iPhone for days while you are traveling)
- Access to all ports of your iPhone 3G / 3Gs
- As a protective case to your iPhone 3G / 3Gs (made with sam material used in iPhone, scratch proof)
- Perfect integration, almost no seams, no longer bulky
- To Sync with your PC / Mac even when the battery is on
- No need to take off the battery even you need to Sync your iPhone 3G / 3Gs
- To control when to have Data HotSync and when to charge
- USB Cable for charging (hidden USB Cable for your data connection and charging anytime,anywhere, and always with you)
- LED Power Indicator

Using dual SIMs looks pretty straightforward. According to USB Fever, you can use one SIM at a time, and you swap between the two by selecting “Home ==> settings ==> Phone ==> SIM Applications: select your desire SIM card.”

Of course there are a few quirks and gotchas when you decide to boldly take your iPhone where it was never meant to go:

Note once you’ve chosen to another sim card, it will have a message “No SIM Card installed”, please wait 1 or 2 minutes, it need sometime to search.

Note:
- When the battery is with the case, the LED will be always on (blue). When the battery is being charging, it will be in Red.
- For sync with PC / charge with PC / Mac, please turn the Charge Button to “OFF”
- This devices does NOT support DUAL STANDBY Mode but you can switch between them.
- Due to complexiety of network compatibility, we cannot 100% guarantee its compatibility with your network, we can only guarantee the compatiblity of the battery part, but if you have probelms of compatibility, we will try our best to help you.
- This battery is more or less same as Another Prodouct that we have, just that it does NOT have DUAL SIM Function.

I suspect that travelers with dual numbers will be happy to give this item a chance, quirks and all.

3-in-1 External Battery with DUAL SIM Function for iPhone 3G / 3Gs