The thing about bags (and trunks of cars) is that the bigger they are the more stuff you put into them, no matter how hard you try to bring only "what you really need."
I carry my 15" MBP in a Timbuk2 and messenger and a Crumpler messenger, alternating for different purposes (one is packed with travel stuff, the other with around town stuff). Both are HEAVY when fully loaded. 5.5 pounds for the laptop itself, plus several more ounces for the AC adapter. Say 2+ pounds for the bag and associated other crap and I'm hauling 8-9 pounds around on my shoulder all day.
I can't wait to retire my MBP for good and slim waaaay down.
I'm focused on the Tom Bihn Ristretto for iPad because it's just big enough for the iPad itself, and very minor amount of other stuff. All in, bag included, it should be under 3 pounds total. It may seem like "not that big a difference from 8 or 9 pounds, but practically speaking it definitely is.
The tradeoff is that in terms of scale it screams man purse, whether oriented vertically or horizontally. But that's something I'm willing to accept.