I spent a half hour working with one in Best Buy today. I've also handled extensively the Olympus E-620 and have the D90. My comments should be taken with the following serious limitations in mind lest anyone draw any conclusions at all:
a) shooting within the confines of Best Buy is very limited (ex. an action shot consisted of a customer moving as if on qualudes between counters)
b) the lens was the kit lens, very limiting, and I couldn't try any telephoto shooting at all
c) I didn't have the time with the salesman being tied up by my time with the camera to really examine photos carefully in playback
d) not being able to switch back and forth between cameras makes some of these impressions very very subjective
With all that in mind and in no order of importance.
A very quiet shutter - nice for candids
Viewfinder very useable but not quite as bright as the E-620 and, of course, not anywhere near as good as the D90
LCD in Live View at waist level both very useful and also slightly limited re focusing and lag time....fine for composing. The LV/LCD on the Olympus seemed brighter to me, slightly faster, and easier to use for focusing. But boy would I not want to swear by that since I didn't use them side by side.
Very light with the kit lens and also felt very comfortable to hold.
As others have said, comparing this and the D90, for all the similarities, seems like apples and oranges to me. The D90 feels and works like a lighter weight
semi-pro (at least!) camera. The D5000 is clearly a consumer camera and it should sell like hotcakes. It's a neat camera. The articulated LCD and quiet shutter still tempt me a bit re candid and street shooting.
Between the E-620 and D5000 if one didn't have an investment in either system to begin with it would be a hard pick in terms of handling. I'd give a slight edge to the E620 for operation, though I have a hunch the actual results with the Nikon DX system will give the D5000 a slight edge there.
Now I look forward to seeing some effective reviews of the D5000.