Nyko has two 3DS accessories coming out on launch day. One is this Power Pak+, which they say will double the battery life of your Nintendo 3DS. It also features finger rests for gaming comfort. The MSRP on this is $19.99. You're going to want that with all the Street Pass functionality this system has.
A week later they're bringing out a Charge Base. This will allow drop-in charging for your new Power Pak+. Each charge base actually comes with the Power Pak+, so no need to buy separately. The Charge Base package will cost $29.99.