My Sophos Anti-Virus v8.0.13C, engine 3.42.10 just warned me about the exact same location (and an additional subfolder) on my MacBook Pro 10.8.3.
I am also running ClamXav (v. latest) which did not(yet?) detect it. Both apps run automated, and this is the first threat I am aware of. My iPhone backups were already set to encrypted.
com.sophos.intercheck: 2013-04-27 17:05:24 -0700 Threat: 'Berserker-A' detected in /Users/USERNAME/Library/Application Support/MobileSync/Backup/26f9c2d0793adf2cc3580dbb0ea9fa6656fa48ce/Snapshot/462db712aa8d833ff164035c1244726c477891bd com.sophos.intercheck: Access to the file denied com.sophos.intercheck: com.sophos.intercheck: 2013-04-27 17:06:08 -0700 Threat: 'Berserker-A' detected in /Users/USERNAME/Library/Application Support/MobileSync/Backup/26f9c2d0793adf2cc3580dbb0ea9fa6656fa48ce/462db712aa8d833ff164035c1244726c477891bd com.sophos.intercheck: Access to the file denied
Sophos app said "see website for manual removal" then said nothing of the sort on the site. The app removal instructions on the site apparently do not correspond to MY Sophos, which I thought was the latest.
I manually entered the folder and could NOT find the "Snapshot" subfolder from the first warning. I deleted the folder mentioned, before I saw info here. I wasn't concerned about the backup, as I can easily make a new one now.