I hear good things about the D40 (isn't there a newer X version?). The D40 replaced the D50, so stick with the newer model. You might also consider the D80 which is a little bigger - here are some reviews I found:
Nikon D40X Review: 1. Introduction: Digital Photography Review
Nikon D80 Review: 1. Introduction: Digital Photography Review
I'd consider the D40x as a good way to start; the D80 is a little bit more.
Another good camera I recommend to all my friends starting out is the Canon Digital Rebel Xt.