http://www.sierratradingpost.com/p/,...s_40L_Backpack

can't vouch for it, but this looks good. removable internal frame, too.
i agree, though, i like my backpack much better than my messenger bag. even with the stabilizer strap on the messenger i still had to wrestle with it to keep it on my back.

edit: i also know kelty makes some nice internal frame packs that a lot of people use around here.