I think web sites should be designed to be as agnostic as possible about stuff like this. I am so often hosed by some stupid javascript that tries to figure out "is it IE, firefox on windows, or safari" that can't deal with my firefox on linux and lynx/links and my cell phone, let alone opera or whatever.
As much as I know I'm bucking the trend, I really, really, really think that TONMO, and every web site, should always produce readable HTML on any device that can read html at all. However, I don't object to using more information when it's available, as long as it doesn't cripple the behavior when it's not available: my Xwindows config is broken, and I want to read TONMO from a text-based browser with no javascript or flash, I want it to work, but I don't mind if TONMO can do this "what size am I rendered in" trick without precluding that sort of thing...
Of course, this also raises questions like: will this get screwed up if I change my window size? Will it get hosed by some http proxies that block sending back the answer? Does it assume I'm not using sidebars and tabbed browsing and such? Will it sometimes get confused and force a horizontal scrollbar on me?
p.s. in some sense, this doesn't really match the original poll question (I voted for "whatever" in fact) because what I really care about is that the content should be decoupled as much as possible from the layout. The best way to do this is sometimes to let the browser decide, which is rather fluid, or sometimes to make the simplest assumption (like fixed width). Also, of course, it depends a lot on how well the vBulletin software has implemented this stuff...