Been scratching me head for a while now as to why my Windows Explorer icons for TortoiseSVN were suddenly missing.
After some digging, it appears that DropBox updated itself and added some more shell overlay extensions.   Since Windows 7 only supports 11 overlays, this pushed the Tortoise icon entries too low in the list.
The solution was simple:  edit the registry hive: HKEY_LOCAL_MACHINESOFTWAREMicrosoftWindowsCurrentVersionExplorerShellIconOverlayIdentifiers
and remove the unwanted registry entries so that the TortoiseSVN entries are in the first 11 found.