An achievement list for Grand Theft Auto: San Andreas has appeared on XboxAchievements.com. While there apparently has been no official word of a SA re-release, I think that this pretty much confirms it. XboxAchievements always gets lists soon before a game comes out, and there are a few other potential reasons for a re-release coming up:
1) The Xbox Originals download has been removed from the Xbox Live store.
2) The 10th anniversary of the San Andreas is October 26, four days from the time of this writing.
3) The next gen GTA V is right around the corner, and since they both take place in San Andreas, perhaps this is meant to be a tie in?
I would be all over a re-release. San Andreas was a game that I missed growing up, and when I tried to revisit it a few years ago, it didn't really hold up very well. All of my complaints could be easily fixed with a little polish though, so if this is a chance for me and many others who hadn't jumped on the GTA bandwagon yet back in 2004, I'm excited. Few games have a legacy as strong as San Andreas.
I'll be keeping an eye out on this one!