I was just wondering are there any apps like XBMC for the 360 available or in development?

NO, until we get a full hack we won't see programs. Flashing the DVD drives firmware is all that can be done.

maybe someone can do it with xna

Although it may be possible, considering the limitations of XNA I doubt XBMC will be coded with it. Not to mention that when the emulators for the x360 were announced and everyone realized you had to actually pay for something (the XNA $100 yearly subscription) to be able to run it on the x360, most people were pissed and refuse to pay for XNA.

If it IS coded with XNA and can be used, it will only be available to those that wish to pay the $100 a year.

Which I don't understand why is so bad, people were willing to pay up to £30+ for mod chips on the Xbox 1. I just don't understand why people are so annoyed that they want to pay for something. You pay £50 for Live a year, £50 for games so £50 for XNA aint to bad. Maybe all these people asking it for free are really falsifying the term backups and pirating there stuff.

The only bad thing about XNA afaik is no network support. Surely a Media Center doesn't need network support.

Network support is vital for a decent XBOX360 Multimedia player. It means your not limited to only 13gb (minus gamesaves etc) of space to hold your movies, sounds, pix.
Microsoft wont allow a piece of XNA software to play back all the media formats we can with an XBOX1 anyway.