I wouldn't mind the idea. The only thing is, I own a PS3, 360, Wii and RECENTLY ventured into the world of PC gaming (bought a Toshiba Qosmio laptop!). I played Dragon Age II on my laptop, because I heard the PC version was so much better.. but I got ticked that I wasn't unlocking achievements on my Xbox. I don't know, I would like the idea better if a "Gamerscore" was a universal thing.
Don't count on it ever happening though, I believe I remember Miyamoto being asked about Achievements/Trophies in an interview, and he said something along the lines of "I want people to play my games because they're fun. I shouldn't have to taunt them with carrots to play them.", and he made a REALLY good point.
So, if anything, I would like stats to be shared with each 3DS/Friend Code (Get every Heart Piece in OOT? The world better know.) The one thing I love about achievements on the 360 is that I can track to see how far my friends have gotten in a game. Enabling a feature like that for the 3DS would be prime for me.