Sequelitis has always been really bad. 6 Mega Man games on the NES, 4 main Sonic games on the Genesis, 500 versions of Street Fighter 2, etc.
But it does seem like some companies are speeding up these days. Activision released Guitar Hero 5, DJ Hero, and Band Hero, all within a few months, and they definitely cannibalized each other (and the whole genre) and combined they won't touch Guitar Hero 3 or 4. Valve normally makes 1 game every 50 years and it changes the world, but they made Left 4 Dead 2 within a year of Left 4 Dead 1 and it pissed off a bunch of their fans. Microsoft/Bungie made 2 Halo games last generation, and they're already making their 4th this generation. Mario got 4 platformers on the NES (in Japan), then 1 platformer each on the SNES, N64, and GC, and now he's getting 3 on the Wii.
But if you look at the list of the greatest selling games of all time, almost all of them are games that didn't get an immediate sequel. So it does make sense.
@cAPSLOCK, yeah I agree that Ubisoft will be remembered mostly for their trash by the Wii gamer generation of tomorrow. A lot of these companies are just pissing gamers off with shovelware and pissing stockholders off by losing money, and insulting and abandoning the fans of the most successful console of all time isn't going to make anybody care when they go down in flames.