The future is bright for these compact mirror less systems. Give them time. Remember this is a new segment and will only get better over time.The NEX's biggest weakness is the lack of high quality native lenses. You have TONS of choices with the D90. With the NEX ... blah.
As far as IQ is concerned, notwithstanding the lack of high quality native lenses, my NEX3 can produce image qualities that are at least as good as a D90. But like someone else said, if you need a viewfinder or flash hotshoe, you are out of luck with the NEX.
Not sure if you have looked through the EVF of a Panasonic G series or not. If you have not tried it out yet, I highly recommend that you try it out. You may be pleansantly surprised at how good they are. After I started using the EVF in the Sony A55 and the Pany GH2, every time I use an OVF (even the pretty good OVF in my K-5), it's like going back to the dark ages.