My mid-70s father and I both have 24" monitors at 1920x1200. I use the full width for most browser windows, but around 2/3 for some. E.g. full width for HN, but it shaves off a bit more than an inch at each side.
My father has a few eye problems and always goes full screen with the text etc. zoomed pretty large using Firefox. I'm more near sighted than he and keep my text zoomed perhaps larger than most.
So one shouldn't assume that if your user has a big screen and resolution that you can use the full width as raw pixels.
Personally, I prefer browser windows to be narrower; roughly 2/3 of a widescreen display. In my case the window is 1220 pixels wide, although the text is probably a bit bigger too.
I greatly appreciate sites that can adapt to the actual browser size. I am frustrated if they fix the layout (especially if they fix the font size, or otherwise corrupt the layout so that changing the font size has no useful effect).
This is a pet peeve of mine. IMO modern 1000px-wide Web design is screwing people with large monitors. If you have 1024x768 then you have one 1024px window — no problem. But if you have 1650x1050 you have two ~800px windows and you have to horizontally scroll. A 2560x1600 monitorzilla fits three ~800px windows, again with horizonal scrolling.
Live columnising vertically is not simple (but it's coming) and so using the width of screen and having a readable line length is actually a difficult design target given the need to also meet the needs of those using legacy and small-device screen widths.
I have a 1650px screen and like being able to put up to windows side-by-side and have most content viewable. Indeed 800px is about the limit of line-length for scanning/readability IMO. On HN I get a 60cols textarea (for comments) of 613px and a comment view width of c.900px; as I said I like a line-length between the two (obviously font-size/spacing plays into this too).
I'm on a 24" monitor at 1920x1200. However, I never ever maximize my windows, especially not my browser.
The latter is because as an interface designer I want my viewport to always be the width of whatever a user has at a maximized window on a 1024x768 resolution, as this is currently our lowest significant resolution used on the website. Also, I feel it's a nice width for consuming content on the internet.
Rather than try and describe my Opera layout (which gives me a nonstandard viewport size), I'll just say that it's this big: http://i.imgur.com/CDIGv.png
When I compare to previous year then the greatest change is 3% drop off 1024x768 between 2008:2009 and 2009:2010; the two leaders remain with 1% of the previous years figures.
My father has a few eye problems and always goes full screen with the text etc. zoomed pretty large using Firefox. I'm more near sighted than he and keep my text zoomed perhaps larger than most.
So one shouldn't assume that if your user has a big screen and resolution that you can use the full width as raw pixels.