If I was Nikon Marketing I would say you should buy a Nikon D90. Here is my reasoning:

1. You are starting a system which will grow with your professional career. The only two manufacturers that count are Canon and Nikon. Sony are trying to break into the professional market, they make some nice cameras but I would go with the crowd.

2. Choose a camera which works the same way as the professional models, has much the same logic and controls, etc. This eliminates the consumer models like the Nikon D3000, D5000 and the Canon Ti1. It leaves you with the Canon 40D and Nikon D90

3. Video. Ugh. Real photographers don't care about video. However local newspapers and web sites may need it when you cover an event. You can't ignore it. This eliminates the Canon 40D

4. Your budget. What you really want is the Nikon D300s with the 16-85VR but it's too expensive. The 18-105VR kit lens is plastic but good and Nikon almost give it away in a kit with the D90. It's a good start.