I do not see the D5100 being an "upgrade" factor to the D7000 or the D90. I think they are close, but still a very large gap.
Main things I look at that the 5100 falls short: (I think my top 4 is where all consumer cameras fall short of the pro-sumer and pro models.)
1) Lack of AF motor - This is huge for anyone with a collection of older glass
2) Metering is still the low 420 area
4) AF Points
Other things that I look at: Battery life, Dual card, Shutter speed, High Speed Sync
To me my top 4 shows how much different the D5100 is from the D90 or D7000 and any pro-sumer camera.
The video has much upgraded items and looks fairly good as does the IQ.
As a working back-up? No - The D7000 would get my nod. As a second, walk around - fun body, I could maybe go for that.
BUT Lack of AF motor!
There are 72 current Nikon lenses. Of the current, 60 lenses are AF and 21 need the AF Drive. Of my 16 lenses, 6 are AFS G lenses. I only have 1 Nikon lens that is not in the current catalog, the 24-85mm AFS 3.5-4.5G (Yep an AF-S G is the only one not in the catalog).