Veritable Avarice, a new blog on moneymaking in WoW, took a break from financial discussion and looked at class representation in tank, DPS, and healing roles by filtering and comparing data available from WoW Popular. Spec population was then checked against class population data available from Warcraft Realm's census and three live realms. Data differences, according to VA, weren't statistically relevant, and he/she is pretty sure that the numbers are at least a ballpark representation of which class is most likely to be filling a particular role within a group.
I play a Druid, so that's really what I feel comfortable commenting on here. While I can't speak to the ultimate accuracy of the numbers, I do a lot of pugging and have to admit that VA's data seems pretty close to what I've seen on my own server. The tank numbers are also consistent with a few things Ghostcrawler's mentioned recently concerning the overwhelming population advantage still held by Warrior tanks, although I wonder whether the Feral statistics are somewhat inflated here by the overlap between Bear and Cat specs. Feral tanks have all but vanished from 5-mans on my server, and it's not uncommon for me to get comments from healers that I'm the first Bear they've healed in months. Less surprising is the representation advantage held by Druid healers. Trees are insanely good in Ulduar, and between this, the rise of the Death Knight, and the de-suckaging of the Protection Warrior spec, that probably accounts for the gradual disappearance of the Bear. Also thought-provoking is just how few Druids hold a share of the DPS pie.
I'd love to hear from members of other classes on the data and how closely it dovetails into their own experience. There's a quick note for Warrior players (or anyone interested in the DPS graph) past the cut, as there's a small mistake on the relevant graph.