While this is an older thread, I thought I'd offer my thoughts to those who are contemplating a similar decision.
I use a D5100 as my travel/lightweight camera. I've got the 18-55/55-200VR lenses as an ultralight option. However, I usually shoot it with a Tamron 17-50/Sigma 30 1.4/70-300VR/SB-400 combination. I get fantastic shots with this combination and have rarely found myself needing more than the camera offers.
Additionally, I picked up a Coolpix P7000. With the upgraded firmware, it's practically the same camera as the P7100 mentioned earlier. It also uses the same battery - which is very convenient. I can stick all this gear in a relatively small bag and walk around with it all day with no back strain. And I always have the option of leaving the D5100 kit at home or at a hotel and just going light with the P7000.