Actually Canon has the larger share of both compact cameras and digital SLRs (I think over 50% in both). Most pros (wedding, sports, news) shoot Canon, and that's been true since the early 80s. At every major sporting event/news event/etc, you can see the ubiquitous red ring (for L designation) and white lenses too (but Sony started releasing white lenses now as well). The tide is turning not only for Nikon but also other manufacturers... there are SO many new choices for DSLRs these days... 2 years ago there were maybe 10 or so bodies. Now there are almost 30 bodies you can choose from (not including old models I mean). Actually I just read an article about how Canon and Nikon (#1 and #2) are losing market share to newer players like Sony.
Canon also offers more first-party lenses. This probably means most to people who are getting into photography because there's more selection and a broader range of prices. Of course Nikon has a good lens lineup too.
However, it is somewhat of a personal preference to an extent. Honestly, the body you have won't affect your photography drastically. For example, no one is going to notice a huge difference between similar lenses on similar bodies from different manufacturers.
Do you mind me asking why Canon over say Nikon? Is it simply a personal preference thing? It seems to me that the Nikon users outnumber Canon and just about everyone else too.
