Well, I always knew it could be done but never got round to doing it. However, I had a few spare hours last night and I spent them setting this up. Using TVersity, a superb completely free application, I can now stream any type of video (plus photos and music) directly to my 360 over my wireless and despite my PC being in another room. Although the 360 only supports a few video codecs, TVersity automatically detects what the 360 can play and if it can't play back a video codec you select, it will have the PC convert it on the fly and stream the compatible video over the lan.
It works a treat and anyone with the facilities to do it I suggest you set it up. I was watching a Xvid copy of Hot Fuzz last night that would never normally play on the 360 without the pain in the arse, time consuming process of conversion.
Oh, and apparently TVersity works with the PS3 too although I obviously can't confirm that.
http://tversity.com/home
http://www.360-hq.com/xbox-tutorials-34.html - A quick tutorial to get it going.