Interesting one here, it was in 2.5.0 but probably not an issue for most people. Testing with my older laptop (MacBook 2.2GHz, 6GB, 500GB Hybrid, 10.7.4) ...
When Traktor is open full screen, switch to Finder, open Safari and then make it go full screen (top right corner 2 arrows), then Command/Tab back to Traktor and you get a quick flash of Traktor and then black screen. Sound and everything keeps playing.
Go back to Safari, change back to normal screen (change back from full screen app), Cmd/Tab to Traktor and you still have a black screen. You need to quit Safari to get your Traktor screen back.
This also happens with my live rig MacBook Air 13" (2011) running 10.7.2 (it's stable so no need to change) with 2.5.0 as well.
This does not happen when you go to Dashboard and back to Traktor (Dashboard is another full screen app or space, always on by default).
My guess is that it has to do with the Full Screen App feature of Lion, basically it's like the old "Spaces" where you can have multiple desktops running different apps. Traktor full screen is more of a Maximise, maybe the Full Screen App feature of Lion should be explored a bit more?
As I said though, not a deal breaker but more of an observation. It's not like any of us need to have Facebook open at our gigs ...
Also noticed that when I play an AIFF for the first time and it analyses it, there is a very slight stutter to the track when the analyse is finished if you are playing it straight away.
Not very often that I'll drop a new track live without listening to it either before the gig or just on Cue in the headphones ... so again not a deal breaker for me but for others that get given a track at a mobile gig and just hit play it might be an issue.