I do think a flash unit will probably be my next purchase. Probably something like a SB-600 Speedlight which I see can be had for approx. £110 on ebay.
Check the power out put, I have a 580EX and it's just about right for all my portable flash needs, I would have wasted my money on the 430EX. I am sure Nikon's are the same, get the very best you can afford.
As for the camera, I chose something that felt comfortable in my hand and was well laid out. I also had two Canon owners tell me to get a Nikon!
I did the same thing to be honest, to me the Canon at the time had a better feel. I haven't picked up anything Nikon has done that is better enough for me to make the swap.
I think it also depends heavily on what other people you know use. Everyone I know uses a Canon, and it has proved very useful as I can borrow or swap lenses and try new things without buying myself. The prime 180mm I talked about above wasn't mine, but after trying it out for a few hours it soon will be

One thing you will need is lots of flash cards. I find I need at least 2 4Gb cards these days. I end up only keeping a few but I really hate deleting from the cameras screen, just a habit I guess.
You will of course need all the cleaning stuff, and I find a polarising filter very useful, I never use UV filters myself. And if you haven't already got one, a good lens hood.
Cheers
Cris.