Some Vista MCE users think they're getting 50Hz with the X360 in HD mode, but no one has a TV that tells them for sure.
I use MCE2005 & just run my 360 in SD mode - it runs 50Hz in SD fine. HD is definitely 60Hz.
There doesn't seem to be any doubt that the 360 is capable of HD @ 50Hz (see some of my previous posts on the matter), it's just a matter of it being enabled by MS. It seems that when it comes to MCX in HD @ 50Hz, the 360 is suddenly just a games console that does some other unimportant stuff too.
Justin