Wikimedia already has a longstanding preference not to split Wikipedias into separate ones for each dialect, as much as possible. Therefore nobody would support splitting the English Wikipedia into American-English and British-English and Australian-English Wikipedias, and similarly a *lot* of effort was put into keeping the Chinese Wikipedia from splitting into Simplified and Traditional Chinese Wikipedias, and efforts to split off a Cantonese Wikipedia have also been consistently opposed.
American, British, and Australian English can hardly be termed dialects. They're better termed as standard varieties of the same language, because each of them has several "acceptable" accents, and the differences are almost entirely limited to terminological differences. (for example, is someone "in hospital" or "in THE hospital"? are they "in the OR" or "in theatre"? did they get food poisoning from "rotten eggplant" or "dessicated aubergine"?
Also, perhaps you've been hiding in a cave for the past year -- the Cantonese Wikipedia's been around for a while now, and the vote on whether or not it should exist showed a clear and strong majority of supporters, while the opposers were a vocal few (read: Andrew Lih).
Also, you might check the two other Sinitic "dialect" wikis created since then:
http://wuu.wikipedia.org/ http://cdo.wikipedia.org/
and of course, http://zh-classical.wikipedia.org/
Mark