Why add extra hardware? Your n900 already has wifi.
Just use X or VNC to share you display between phone and HTPC.

Seriously, I thought the video-out is just for travelling, e.g. hotel rooms.
Why would you use it at home when there are so many better alternatives? If I want to watch a video that is on my phone, I just connect it to the TV computer with USB mass-storage, and use the TV remote control.
It would be nicer to run a UPnP server on the N900 and stream to the mythtv client though.