A dedicated line means that you don't stand on the same line as those with Priority AAccess. You walk up, hear the three beeps and go directly to TSA Pre. No airport has that... not even LAX. Presently, everyone has to queue up w/ the Priority AAccess folks, which is fine, but the line can get quite long at hubs like DFW, MIA and ORD. What I am saying is that you will not see a TSA Pre line that is separate from the PA line for some time.
Yes, LAX does, like said above, sometimes they don't allow it for some reason, but if you are TSA Pre, you walk up to the left of the Priority AAccess lane, get let through the rope, and approach from the left side, which is primarily for TSA Pre only (and has always been empty when I have approached it).
LAS separates you before you get to the Priority AAccess line, and if you are TSA Pre (I think they actually scan the BP there), you get sent down a separate lane, which is only for TSA Pre.
So while from what I've read all other airports have a shared line, LAX (at least some of the time) and LAS have a separate line for TSA Pre.