When new real estate photographer ask me for DSLR recommendations I always recommend Nikon or Canon. I notice that the same thing usually goes on, in the PFRE flickr discusion group. The usual reason is that with Nikon and Canon you have more choices of lenses.
Now days there is another reason to stick with the big two camera manufacturers. It is as Thom Hogan says in his 2009 predictions, "the worldwide econopanicolypse". The problem we may to face is there may be some camera manufacturers that don't survive the worldwide economic stress we are undergoing. Here is what's happening:
I'm not as bold as Thom, who is predicting which companies are going under, but I think if you are thinking about purchasing a DSLR system soon, it doesn't take a rocket scientist to see the potential downside of owning a defunct system that's difficult to get support for. I'm not saying you should switch systems, I just think it's not a good time to be making a new commitment to a non-Nikon or Canon system. Some of these systems may not make it through the economic hard times.