1140? 1280? C30, C60, C90, go! The issue, for me anyway, is that you shouldn't design anything that is meant to be read for a wide display. You can, yes, it's certainly easily doable. But it's bad design to make text difficult to read.
You can't make a hard and fast xxxpx rule, because different resolutions will display those pixels differently. But pick up any random book and a ruler and you'll see that the text is typically not more than 4 or 5 inches wide, depending on the typeface and size. When you make a line longer than that it becomes difficult for a person to read.
I have a very wide monitor here, and I have to open my browsers in about half of the available space, because people use fluid design where they shouldn't. Since I assume we're talking about forums here, and forums are all about reading, it seems like a worthwhile mission to get people to turn away from fluid layouts.
Remember: when you use a fluid layout, the terrorists win.