Try logging out and logging back in. That worked for me when I had that issue.
I am also experiencing issues with trackpad gestures on my MacBook Pro running DP5. The 4 finger swipe up/down gestures can stop working at any time and the only fix for this is logging out and then back in. A rather strange thing that happened yesterday was that the gestures came back to normal without having to log out (used the laptop for about a few hours having the gestures broken). I have no clue how that was possible though ... Maybe (hopefully) it's just some random bug introduced in DP5 since I also had no issues at all with gestures in the previous betas.
The same, the gesture came back once yesterday and i guess this is related with jitouch. Do you have this app on your Mac? I've done some tests on that, when I boot my machine without jitouch auto-launch, the gesture will not fail.