http://www.escapistmagazine.com/forums/read/18.372783-Poll-Sub-or-Dub-Anime-Survey?page=3^ 56% state preference for subs in general, and it's a non-anime site, so sample of general online population
http://myanimelist.net/forum/?topicid=670637&pollresults=1^ 56% state preference for subs (but in reference to the specific anime Yugioh). Myanimelist is an anime-specific site, so that could bias things. But it being a very mainstream series probably unbiases things. An ultra-popular shonen series like Yugioh is probably a good barometer of "Joe blow" anime-watcher.
http://www.gamefaqs.com/boards/676844-naruto-shippuden-ultimate-ninja-storm-3/65583799^ of the people professing a specific preference, 30 went with Japanese audio, 20 with English audio. For another ultra popular series, Naruto.
Hell if dub watchers can't even pull 50% of Naruto and Yugioh fans, then I doubt they make up a majority of viewers of the less-well-known series in general. I'd say that's only true of a few specific series that are known for having great dubs.
More polls:
http://www.animenewsnetwork.com/bbs/phpBB2/viewtopic.php?t=160185^61% Subs, though it's from anime new network
http://myanimelist.net/forum/?topicid=691643&pollresults=143 for subs vs 28 for dubs (not counting "I like both"). Which is 60% for subs again.