Based on this thread, they have a "test" P25 system up at his time, but regular traffic is still on the current analog system. Long term, from what I've seen noted here and over on the DFWScan list, a conversion to P25 is planed, but still several years away from full implementation. (I don't have the link to the 'plan' habdy at the minute, but I believe it was by 2014 or 2015. Somebody else may have a better timeline.)
The "switch" earlier this year was the installation of the P25 system controller for the new system (for initial testing & evaluation, probably). Actual conversion of the existing system, as I said, is gonna take some time.