I guess I'm used to "console sized" UI's, after Oblivion, Mass Effect 1/2, FO3, Borderlands, etc, so it doesn't seem all that "clunky". But I do enjoy using resized-UI mods like DaRN.

19.10% of steam users use 1280x1024. 18.23% use 1680x1050. (full chart http://en.wikipedia.org/wiki/Display_resolution.)
Interesting. ~16% using 1920x_____ res, and 3.4% using "other" (which seems to include all single-monitor setups above 1920 wide.)
Personally, I ran 1680x1050, but wish I could have gone lower - my card is on the weak end, and a lower res might have allowed me to push some of the distance sliders up a bit more. Alas, FO3's lower 16:10 resolutions were bugged, and instead showed something like a 4:3 res, squished into a wrong aspect ratio, and letterboxed. It was very strange. Hmm, wonder if that ever got fixed in one of the later patches....