When the iPad was about to be released I wrote some of my expectations. I can't find them now, but largely they centered on the iPad being a media consumption device, leaving the computer for media creation.
Obviously, I was largely wrong about that. The iPad, with a keyboard, would, I think, be a perfectly acceptable main computer. I wouldn't do it, but I think it would be livable.
But what I wanted to talk a bit more about is Plex. Plex, once you install the iOS app, is a fantastic media center application. You run the main server on your computer, and it can serve out your content (music, movies, photos) to your iPad. Sweet.
It even takes care of transcoding, where needed.
Even better, it builds a nice, browsing interface, which is especially nice for your movies and TV shows. It doesn't recognize everything, but does get an amazing amount of stuff right.
I've been playing with it for a couple of weeks, now, and the only problems I've run into are some foreign stuff where it can't find the subtitles. That's definitely annoying, but it's a small subset of my foreign stuff (and I've got a LOT of that).
The other cool thing you can do with it and an iPad (and I haven't done this because I don't have a computer in my theater yet), is that you can use the iPad as a remote control for content playing on the computer.
As I said, it rocks. And if you want to watch your content anywhere, it's a cool way to go about it.