Obviously, I dont know your limitations in size and weight etc. but I have two D300's (one for normal, no hazrds to the camera and the other for go anywhere, dust times) but more importantly, I have a D5000 (handy articulated screen for macro work and going down low or holding it high in a crowd) which is more or less the same size and weight and it works great with my SB-600.
I am sure everyone here will agree that Nikons build quality is spot on, Olympus on the othr hand, the wife bought a compact before XMas (even though I nagged her to get a Nikon) the build quality was awful, I had seen a better screen on a kids digital toy computer.... sure enough, she exchanged it for a nikon.
Now I know it was a compact, but, the name it carried spoke volumes to me and even though I am a Nikon man anyway, I wouldnt change, not even to Canon after the Father in Law's 350D circuit Board failed after a year and cost him £150 to get it repaired.
Stick with the D5100, the controls are easy to use, you have the Expeed 2 engine inside and a 16.2 MP CMOS sensor and the articulated screen that can be used for high or low work without doing harm to your back.