In reply to The Lemming:
I picked up a Nikon D90 about a couple of months ago to replace my aging D50. Compared to the D50, it's light years ahead and I'm absolutely stoked with it. I have very few complaints, overall, and would heartily recommend it to anybody considering a "mid-range" dSLR.
According to camerapricebuster, there's only a £100 difference between the d5000 and d90. For me, this would not be a deciding factor when purchasing a £500 camera. The only advantage for the D5000 is the flip out screen, but it trades off handling, lens versatility, the gorgeous lcd (a real bonus if you chimp a lot) and maybe the viewfinder. I would imagine the general build might feel slightly better on the D90, as well.
That said, they will probably take fairly similar pictures, given that they have almost identical internal components and software.
Hope that helps!
J