I totally agree with the posts about readability on wide monitors. You can only read so far across before it becomes too much.

To ease your concern, I'd check the stats on who's viewing your sites, and what the average size/resolution of monitors they're using.

If you find that there are a lot of people looking at your sites with some ridiculously huge resolution, perhaps you need to redesign...if not, don't worry about it.

I personally build most of my sites to around 950px width. I just prefer the control I have over a fixed width site.

Hope this helps.