"Professionally" is rather an extremely broad area with levels that go from low to very high, with each one requiring a different quality level of equipment. You also need to consider the direction that photographic technology is taking and whether you should buy now or wait a few months for new models and important improvements.
In super simplistic terms Sony is the least expensive with $1500 getting you the A77: a 24 megpixel camera with some great innovative features or pay $4,000 in April for the Sony A99 with 36 megapixels full-frame with even more features. With new Zeiss and Sony G lenses that would give you quite a good quality base for some kinds of pro work.
The Canon and Nikon top pro cameras are more expensive but for studio work, some sports and bands they are very popular. For weddings, engagement photos, seniors, family shots, public relations, informal portraits, scenics, etc. all 3 brands: Sony, Canon and Nikon will do a great job.
skieur