Havent you just sat next to your big HD telle while using video out on your N900, and thought.... "How much nicer would this be if I could sit on my sofa while using this??"

Well now you can!
I spent quite a while looking at the cheapest ones and found these

These types of devices have been out for a long time, but just thought i'd make this post incase people weren't aware.
They go by the name of:-
AV sender / receivers or AV transmitter / receivers.

You just plug the receiver in to your tv via scart, and use the component nokia tv out cable to plug into the transmitter.
And bingo! You got wireless video from your N900 to your phat HD TV. :-D

(Note:- You aren't truely wireless unfortunately, as the transmitter does need a wall socket for additional power. But it does mean you can sit in comfort to use the N900 on the tv :-p)

If anyone can find a battery powered transmitter that would be something special.

That can be really good work-around . The only question left is how's the quality for both video and audio . Actually , I have been thinking about a future device which is pocketable . You carry it with you all the time as a mobile device on the go . When at home or in the office , just simplly plug into some big screen with comfortable keyboard , then a office desktop or home entertianment center will be there . N900 is the device has that potential .

UPDATE!

Just found this little beauty here
Only 1 device needed as it transmits via UHF, so just tune your tv to pick up the signal. Alot cheaper and smaller

I just use a twin tripple (2xaudio + video) phono extension lead. No power needed, no quality reduction... dead simple.

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.

Had a couple of (similar) those to send video from a pc to the tv - quality was crappy. First, if they are 2.4 Ghz wi-fi traffic will add lines on the image and clicks to the sound. Second, even with the PC and Wi-Fi off and them side by side the quality was still unimpressive. And while having a cable+power brick I would preffer one cable to the tv.