If they're still in your download history you can download them again just fine, but things that are completely removed from the marketplace show as "Uknown Game / Unkown Offer" in your download history and can't be redownloaded.
I've yet to see that happen to an actual XBLA game, but I've seen it happen with pics, themes, and Indie games.
"Hey Sockbaby! You wanna food up with us or what?!" Game o' the Moment: Hannah Montana - The Movie Game