They are usually used for charter. There are some threads on here with more information, but basically they are used almost entirely for military charter. The ones you see at DFW
are almost always military charter, often coming across the Pond from Europe and points farther downstream. They fly basically the same mission that the North American 767s do into DFW
I say "almost" because the new, smaller ATA occasionally needs to press them into service to cover a scheduled flight, either due to equipment issues or to take advantage of exceptionally strong demand, say around holidays to Hawaii. I have also seen them on a (very) few commercial charters. Apparently, the ATA commercial charter department is getting revved up again, and I can only suppose that either they are allowed to sell the L1011s occasionally, or they sold a 753 that then went tech.
As you may know, two of the four L1011s were "D" checked at GAMCO recently, one at the end of last year and one at the beginning of this year. The other two are due for D-checks in 2007, and a persistent rumour has it that ATA will bring pre-owned aircraft on property before that time and so will park the un-checked L1011s before their next D-check. The rumoured aircraft are some of the about-to-be-retired NW
DC10-30s. It has long been said that the 767-300 is probably the optimal aircraft for ATA to replace the L1011s with for military and commercial charter (2 engines, 2-person cockpit, many common mechanical systems and common type rating with ATA's existing 757 pilots, etc.), but despite an ongoing effort, ATA has not been able to find those aircraft at an affordable price. Wait 5 years and things will be totally different as 787s and A350s (or whatever) come on line, but for now...they can't find 'em.
Hope this helps.