There's been discussion of a Pokemon MMO for as long as I can remember. The game is perfect for it, but the problem is Nintendo is a real snail when it comes to catching up to the rest of the world in online capabilities. Yes WoW is dominating right now, but that's on the PC. There's little to no competition on the console markets, and besides it Pokemon even if there was competition it'd do VERY well anyway. I mean come on, it's the 4th generation of pokemon and they've changed so very little about the game, yet people STILL buy it by the millions.
I doubt they'd do anything with monthly fees, all the other online games for Wii, DS, and PS2 and PSP and PS3 are free, I really don't think they'd make you pay to play one game and not all the others.