It doesn't matter if your game is written using the XNA Framework or its a C++ native DirectX game. The process for getting onto Xbox Live Arcade is the same. You must apply and be accepted into the XBLA developer program. (see below for 'You Tube for Games' comments')

Start here: http://www.microsoftcasualgames.msn.com/developers_xla.htm

Read these:
http://xbox360.ign.com/articles/771/771387p1.html
http://www.gamedevblog.com/2007/03/some_answers.html
http://forums.xna.com/thread/10376.aspx (for information on GSE 'Pro' type features that XBLA developers have access to)

Also check out the slides in the casual gaming track from last years meltdown "Developing Games for Xbox Live Arcade" (from http://msdn2.microsoft.com/en-us/xna/aa937787.aspx)

Bear in mind that games are not launched very frequently on XBLA and there is a lot of competition. So make sure you have a solid idea and preferably a good demo before you think about this. Also note that there may well be a financial commitment to you for access to dev kits, developer support and the certification process.

Doing your demo using XNA GSE does give you one big advantage - you know your demo will run on the 360 and there will be minimal code changes to get it working later if you are accepted. Compare that to doing your demo in flash and having to rewrite, or in C++ with no knowledge of the XDK.

"You Tube for Games"/"Xbox Live Community Games"
After much speculation the rumoured 'you tube for games' was announced at GDC 2008 under the new title of "Xbox Live Community Games". Its an alternative way to get your XNA GS games in front of people without a creators club subscription. See Xbox Live Community Games FAQ for more information.