I reckon you can't go far wrong with any of the big names – Canon, Nikon, Pentax, Sony…
The former two probably have the edge in the number of lenses and accessories available, but if you're just starting out that's no big deal – you'll go a long way on the kit lens which comes with any entry-level model. Once you become a bit more discerning, you can start to add prime lenses and quality zooms and build to a second body from the same system.
My basic advice would be to go for a basic model with a decent starter package (a couple of lenses). Any SLR will have all the basic features you'll need and encourage you to explore modes beyond "auto".
I bought myself a basic Sony SLR a couple of years ago and I don't feel I've outgrown it yet.
http://www.dpreview.com is a great source of info.