Even if the reports say screen resolution is 1024 wide, that doesn't mean the browser window is this wide. As been mentioned, many people have their favorites attached to the left or right, which I've also seen enough of to know it's pretty darn common. The other situation is that many people simply cannot stand to read something that wide. 800 wide is a pretty good representation of an 8 1/2 wide printed sheet. It's comfortable to read, not overbearing with too much information, and simply works. People are used to this format and know how to quickly scan it and locate what they want.

The problem with using up the entire 1024 wide space is that way too much information gets shoved down the gullet all at once and it's overwhelming to figure out what to read and where to click.

Fluid is the absolute best method for a website, but it simply won't work for every design out there. You have to plan a fluid site properly and respect the fact that certain things cannot be done yet. You also have to understand the concept of letting your page break gracefully. Some people would have a massive heartattack if their precious layout was a pixel off, where a fluid creator would be thrilled that it's only one pixel off.