After upgrading to the latest version of TortoiseSVN (22.214.171.12495), it's context menu is no longer available.
When attempting to run it manually, I get this error:
The application has failed to start because its side-by-side configuration is incorrect. Please see the application event log for more detail
The application log shows this
Activation context generation failed for "C:\Program Files\TortoiseSVN\bin\TortoiseSVN.dll". Dependent Assembly Microsoft.VC90.CRT,processorArchitecture="x86",publicKeyToken="1fc8b3b9a1e18e3b",type="win32",version="9.0.30411.0" could not be found. Please use sxstrace.exe for detailed diagnosis.
I remembered I'd seen this thing before just after posting to SO
It seems that later versions of TortoiseSVN are built with Visual Studio 2008 SP1 (hence the 9.0.30411.0 build number)
Installing the VC2008 SP1 Redistributable fixes it
Confirmed working on windows 7 x64.