hey, imagine for a second if nintendo's next true gameboy succesor could read these same SD cards... and were wi-fi enabled... you could potentially be playing a game on your rev, then do a real-time save, pop that card into your gameboy, and pick up right where you left off as you're waiting for a bus or a gf or during your lunchhour... obviously, the next gameboy probably won't be powerful enough (or the 512mb card big enough) to realistically transfer and run an entire game world, but it could possibly work with one or two levels--at least the level/world that you're currently in...
i think iwata or someone said something about being able to seamlessly play the same game on DS and rev in this way...
actually, even if you couldn't download a level to the SD card, it would be cool if you could download the game music to the card, and then stick the card into your mp3 player...
or maybe the SD card gives nongamers a reason to use/own a rev (ok, i'm obsessed with nintendo's nongamer strategy)... maybe since the REV is wi-fi/internet enabled, the SD card can be used to store bookmarks, emails, and other bs for nongamers... it would be like the harddrive needed for their internet/email crap. and the rev could be their quiet, affordable, (what was the third thing) console that nongamers could have a reason to own...
heh. Reggie's comment about what kind of device/controller can play nes/snes/n64 games.. a keyboard (emulators) comes to mind.

hell, with an SD card reader built-in, the REV could even function as an easy way to display pictures and movies taken on digital cameras( that use SD cards as the medium) on your tv...
...as far gaming functions using an SD card, i have no ideas... aside from saving pictures of your face onto the card using a digital camera and then using that image in a game... and of course, simply being able to supply your own soundtrack to a game (the way xbox does... or was that quake or unreal that let you do that?).