If you're saying the PSP will have PS2 ports:
Yes, it will. It will also have plenty of original games, too. The GBA has a lot of ports of NES and SNES games, but the overwhelming majority of them are original titles. It wouldn't be too far-fetched to say that a big chunk of the PSP's launch lineup will be ports or games from familiar franchises, because you aren't going to easily buy a (possibly) $40-$50 game if you don't know what you're getting.
A few of you are bringing this up, but it has nothing to do with the PSP's battery life.
If you say you'll be fine by plugging it in:
While it's true that you, as an individual, might just plug it in to the wall all day, the fact is that Sony will be marketing this thing as portable, (hence the second "P" in PSP) meaning you can take it wherever you go. The vast majority of people who will be picking this thing up will be using it outside of their home (or otherwise not plugged in). If you can only play games on it for three hours at time, that means you're going to have to charge it up every time you're not using it. This can start to become a hassle, like if you want to take it out, but can't because the thing isn't charged.
You don't buy a laptop or iPod to use exclusively at home. You take it out with you. Don't think the PSP is any different because it'll play games. You might notice, though, the iPods and most laptops have great battery life. Think there's a reason for that?
If you say you can just buy an extra battery:
Have fun paying another $100 on top of the $400 unit and $50 games; i.e., it won't be cheap. Also note that the SP doesn't need an extra battery because of how long it lasts, and the "extra batteries" for all other Game Boy models are AAs and AAAs. Cheap.
If you say different games will take up less battery juice:
You do not know this. No one knows this. Kaz doesn't even know for sure, because he had to take a shot at a "realistic" figure. While it's okay to speculate, trying to justify being okay with the poor battery life by saying this is wishful thinking. Here's what we do know: Disc media needs to be spinned, and spinning takes up a constant amount of power. Today's games either have load times or stream it off the disc to cut them down. Either way, that disc doesn't move by itself.
While it's probable that a game like GT4 would take up a hell of a lot more juice than something like Tetris, don't forget that a hell of a lot more people will be playing GT4 than they would be Tetris.