I've already contacted them about it. The number is 18002553700.
They ask for your serial, and then you tell them your L or R buttons are messed up. In my case, it's both. I'm almost certain it's hardware, but it could be the use of a 3-party device.
Anyways, they ask you if you want to be sent a box and a free shipping label, or they route you to the closest repair center, which in my case was 45 minutes away. They claim that once there, your DSi will be repaired on site and returned that day.
Hope this helps.
Good luck!
BTW, when you start your DSi, you've linked your DSi to an account on the Nintendo server. By downloading DSiWare, you save it to your DSi's system memory. When they goto repair it, they format it all. Doesn't matter if you paid for the points or if they are free. They wipe it. Also, don't leave your SD card in it.
Check the Nintendo forums if you want more info.