OSX is getting a new App Store with the release of OSX Lion.
Does anyone know if Unity applications will be accepted?
Yes, Unity apps can be prepared for the Mac App Store.
See instructions here: http://technology.blurst.com/unity-games-and-mac-app-store/
Mac apps have to be signed for the store in XCode, just like iOS apps, so no, Unity apps won't be accepted as-is. Since Unity can already handle XCode projects thanks to the iOS publishing, possibly that could be adapted to allow Mac store publishing as well.
BTW, the app store is not for Lion; it will be started within 90 days for Snow Leopard.