I carry a relatively small internal frame pack on all but the shortest hikes. I used to strap my tripod vertically to the outside of the pack, but when I switched from a Gitzo 2227 to a Gitzo 1258 that became less convenient. Coincidentally, the smaller tripod will fit entirely within my pack (with head attached). That's what I do now.
I used to strap my tripod horizontally under my pack, but that didn't work so well when I switch to Gitzo tripods (the locks on the Manfrotto tripod kept it from slipping out of the loops). It also put a lot of wear and tear on my tripod. I figure that's what tripods are for, so I didn't sweat the nicks in the anodizing too much. It would also snag on trees and such, which was annoying until I learned to maneuver around it.
If I had it to do over again, I'd buy a backpack and a tripod bag at the same time, with the specific intention of strapping one to the other. You have to make sure your backpack has appropriate anchors for holding up the tripod, and that your tripod bag has some way to make use of those anchors. Otherwise you're not going to have much luck. Make sure that the part of the backpack you're anchoring to is supported. If having an empty pack means your tripod isn't supported you won't want to take it with you when your pack isn't full.
Having my tripod inside the pack makes it a little less convenient to get out, which, I'm ashamed to admit, means I don't use the tripod as much as I should. Plus, if I've been wading in a stream the tripod legs may have filled with water that will gradually seep out. Much better to have that outside the pack.
