So I learned a few things today - and I want to point them out because I have yet to see a review that talks about this caveat.
For all those of you excited that the DSi finally supports WPA2 encryption and look forward to playing your current Wi-Fi enabled games online over that method. You can go back to being disappointed.
So I just got done with a little experimenting today on a friends DSi unit. Looks like in order to get WPA support the unit pidgin holes the settings into slots 4, 5, or 6 instead of the normal 1, 2, or 3 that we've had in the past on the normal DS units. Getting to the shop and any other DSi online apps worked fine with the WPA settings in 4. However, booting up Mario Kart showed that we were unable to connect to the net.
I think I know why this is the case, and if I'm right, it'll be unlikely that older games will ever support WPA and that you'll have to wait for newer "DSi" games to get that capability.
Nintendo knows how to make networking a PITA.