In my experience, the bit that you'll really notice about carrying a backpack is the other cruft besides the laptop. I carry my polycarb MacBook to and around Uni basically every day, and with only it and a bit of stuff in the bag, no problems. It's when there's lots of books or notes to carry too that I start to notice it.
And my personal viewpoint on carrying 5lb notebooks - I've not been able to carry very heavy bags since I sustained a back injury a couple of years back. Which is where the 13" MacBook really shines as a full-featured, but utterly manageable, notebook. I wouldn't say that there is a serious problem with the weight unless the owner has back troubles, and that you also feel a lot less of the weight of something in a backpack than in your hand.
And yes, the Lenovos are even lighter than the MacBooks, but sadly they run Windows, which is a total dealbreaker.