numnum - 7:37 pm on Feb 4, 2012 (gmt 0)
Go for center or a fluid design that takes up all the screen (or more ideally, all the width up to a maximum so it doesn't look silly on large monitors).
I align center at 928px (or 948 with wrapper) width (not fluid). This width fits the iPad landscape orientation perfectly, and it's just about right for most laptop, notebook, and even desktop users. Users with a high-res or very large screen see some white space to the left and right, but that's fine with me. The alternative is a fluid layout, which can make for pretty ugly experience for these users. (BTW, I don't bother (yet) with alternative style sheets for different devices.)
If you align center and limit the total pixel width you're assured that most users will at least notice an Adsense or other ad in a left or right column. That said, ads embedded in or immediately below content always yield the highest CTR. Agreed?