To Catgrr: I agree with you totally, however I would like to suggest that you don't verbally slaughter the forum infants, at least not all the way to bloody stumps. Have a heart <3.
Anyway, as to the original post, I would like to pose a counter-question, something to the extent of "Maybe games aren't getting shorter, we (and by that I mean experienced gamers) have just gotten...better?" I know the concept at first sounds like a scape goat for game makers, but think about it. The more games we play, the more puzzles we learn to solve. Those same puzzles have been repeated throughout gaming generations (such as find a block, move it to a button, push.) The reason I bring this up is that going through my favorite N64 game of all time Zelda: Ocarina of time I ended up getting through it in 8 hours, and hardly remembered any of it. From my knowledge of zelda, or the mario series, or half life, etc...I can pretty much see an obstacle and have a pretty good idea how to solve it right off.
Our resources for beating games, such as online strategy guides, faqs, forums for puzzles, etc. have multiplied many times since the N64 days.
I think it's a little of both, truthfully. Next-gen producers produce slightly short titles filled with puzzles that are, for the most part, intuitive.
Many cheers, Unholykrumpet