If you want your game to appear on Xbox Live Arcade, you probably won't want to have it on a Nintendo or Sony online store first. Talking with Eurogamer, Chris Lewis of Microsoft stated that have the right to refuse
to release a game on the Xbox 360 if it's released simultaneously on their platform as well and have the same content as others. This applies to Xbox Live Arcade as well.
Now, if you're a big name title, you'll get by for sure. One title that quickly comes to mind is Batman: Arkham Asylum
as being able to play as the Joker was only available on the PlayStation 3 version of the game. If you're smaller or independent and hoping to get your game on the 360 as well as PSN or perhaps Nintendo's online store, you'll probably want to release it on all platforms at the same time.
Sony seems more relaxed on this policy, which should be the way to go. A game shouldn't be refused on a system just because it debuted on another one before.
Microsoft can try to defend this all they want, but really who thinks this is any good besides the suits over at Redmond. The two quotes that make me laugh are, "we seek to maximise our own advantage to ensure the playing field is even, and certainly plays to our advantage wherever possible
" and "we are able to offer things other people can't offer, that allows that exclusivity and unique elements to it that might not otherwise be available elsewhere.
There are many things that Microsoft does right, but also many that they do wrong. This is one of those things that's just wrong.