You have the IPs right and subnet mask also I take it. Do you have firewall enabled in DS that might be blocking media communications?
I have no other ideas really, if all else fail in situations like that I do the restart dance. Turn off and on everything that can both hardware (tv, ds) and software (media server; stop/start, reinstall if need be), reset everything (media server device list for example), use logging where possible (media server has Diagnose, dms.dmslog is a gz file).
DS111 (DSM 4.2) and DS109 (DSM 4.2), both connected to gigabit ethernet and wireless lan.