Still, I wouldn't get too fixated on the notion that your daypack MUST attach to the main pack in some fashion. That's not the case. Any ol' daypack will do, from any makerin almost all cases, you can easily stash one inside the main pack, or roll it up and lash it on the outside.
I don't doubt you'd be happy with an Osprey pack, but if you think outside the box you could get into something a little more rugged, such as the Gregory Lassen ($299; www.gregorypacks.com). This is a well-designed pack with a nice slim profile and 4,500 cubic inches of load-eating capacity, plus great suspension. To go along with it, get an Eagle Creek Packable Daypack ($30; www.eaglecreek.com), which holds all you need for a jaunt around town and will stuff into any old corner of the Lassen. The same goes for any pack/daypack combo. But, as you're drowning with all the options, stick with the Osprey/Osprey combo. A nice pack and mini-pack at a good price.