Dead Rising 2: Case Zero is the number one seller on Xbox Live Arcade. The performance isn't too surprising, given the low, low price of $5 and estimates of the game's staggering download numbers. Also, Case Zero lets you maim zombies with a pitchfork-shotgun, which probably encouraged sales.
On the other end of the Live spectrum lies Sonic Adventure, the first in a new series of Dreamcast re-releases, which didn't even make it into the top 10. The first week of sales is generally where most XBLA titles perform their best, but it seems even Sonic's super speed couldn't get him higher than 15th. Then again, Sonic Adventure is essentially a straight port of the original -- bugs and all -- so Sega probably didn't invest too much money in the port. It's also worth noting that the other two releases last week, KOF Sky Stage and Space Invaders: Infinity Gene, didn't crack the top 20 at all.