I'm aware of it and all my mods are 100% XHTML 1.0 Transitional and CSS 2 compliant (and the release thread contains such an icon)
But in fact it isn't that important because vBulletin is working with tables, so it isn't semantic (and that's the biggest point of the W3C, not a lost alt attribute). Although I report W3C errors to other coders when I use their mods and I find some.
The checkbox for 100% W3C compliance would be a good thing IMO :up:
But maybe there could be more checkboxes added: fully phrased, ...