95% of all the wedding photographs that ive ever edited for other wedding photographers have been Canon..
i think i know 3-4 wedding photographers who use Nikon systems, one just switched from Canon to a Nikon D3 because he wasn't happy with the focusing of his 1D3.. and he's made a $12,000 investment for the change..
i think alot of people get caught up in how many more focus points a system has, all the extra buttons etc, weather sealing etc.. ive had both my 20D's out in the rain with no ill effects to them (ok wasn't torrential)
i also know photographers who previously owned a S3 who moved to Nikon D200's rather than going S5..