May happen with NERO, but it definitely happens with WMP11.  I've got 4 of these machines and only one is running wirelessly.  It is the ONLY one to have this problem.  I have not tried this recently, but when I first got them I didn't have WPA security set up.  I don't remember having a problem at that time.  However WPA is now set up and this problem creeps up a lot.  If you start a movie then stop it shortly after it starts (minutes into it), then usually it will work and the list is not "greyed" out.  If a movie has been playing for a while or it runs to the end, you can be almost certain that it will lose connection at that point.
-Jack
P.S.  Another problem I've had since DAY 1.  Using WMP11 (doesn't happen with twonky) when the DSM plays for two hours into a movie the machine loses connection to the server.  I've gone back and forth with tech support a while ago - many messages - no resolution.  Movies ARE encoded correctly.  Example:  If a  movie is 3 hours long and you start it 30:00 into the movie, the dsm510 will disconnect from server at 2:30:00.   If you start a 2 hour movie, then pause it for 30 mins, DSM510 will disconnect from server at 1:30:00 into movie.
Again, does NOT happen using Twonky - only wmp 11 (which I must muse for it's rating ability).