I feel your pain. Within a month and a half, Guitar Hero III, Medal of Honor Hero's 2, Mario Galaxy, SuperSmash Brothers. All of which I must own.
And there's a bunch of other good ones I'm eyeing up like Resident Evil: Umbrella Chronicals and Battalion Wars 2.
And on top of everything, outta nowhere Zack & Wiki is really good so I had to pick it up.
I'm just glad I only own one system. I can barely keep up as it is.