This needs some more details. I can already run my iPhone 3G for 24 hours without external batteries.
Do you mean 24 hours of talk time? Of 3G browsing? Of gaming apps?
It sounds like a great product, but in order to hook people in, you need to have a chart of some kind labeling how long it takes to charge the external, how long it takes for the external to charge the iPhone, and the usage.
If you provide concrete details on how effective this battery really is I'll definitely buy one. But I'm wouldn't buy a product that requires 3 batteries to give me 24 hours on my phone when I get that already without the batteries.
Throw us some details, we'll throw you some money.