Hi Rory - I shoot Nikon, just started "birding with camera" 2 yrs ago with a Nikon D40 and a 70-300mm VR zoom. The D40 has been called "the gateway drug" to DSLR's LOL It takes great pictures and I have been very happy with it - you can see some of my pics here:
http://www.flickr.com/photos/jmcda/ We have had Nikon cameras forever and never been anything but happy with the brand.
However, now that I am hooked totally and completely I need
more and will be buying new gear this year. I have not read any really good reviews on the D5000 so far - the D90 is a better buy and a better camera apparently for less money. I am looking at a D300 just for the durability over the consumer grade, but I might end up with a D90 which has similar features, depending on which one feels more comfortable in my hand. I am not interested in the video ability - if I want to shoot shorts I will buy a camera made for video, and I really don't like that swivel screen on the D5000 - I wouldn't have time to use it.
I am conflicted on glass as well - would love a 500mm prime and a teleconverter but I think it is too much for me to carry around all day so not sure what I will end up with - been looking at the Sigma 50-500, not sure about quality as I have only seen pics from one person I know who has it and the "other end of the lens" might be the problem as her shots are grainy and not so good.
I will probably go for a 400mm and a TC as more manageable for me but I can't decide.....I would have had new gear for this spring if I could make up my mind! LOL Whatever lens you get it won't be enough...never enough
(lens envy is a common malady this time of year, along with warbler neck and birders backache)
so if you want Nikon 'cause its the best (I'm going to start a war but I'm leaving for more birding) :-) I would look at a D90 and either the 70-300mm VR zoom or the 80-400 VR zoom.
Joann
As Mathew said, it is a pricey hobby but the challenge is so much fun!
Joann