It appears that I can no longer step into run time library calls. This used to work before SP1, now it only prints, e.g.:
'thread_test.exe': Loaded 'C:\Windows\SysWOW64\msvcp100d.dll', Symbols loaded (source information stripped).
'thread_test.exe': Loaded 'C:\Windows\SysWOW64\msvcr100d.dll', Symbols loaded (source information stripped).
Searching using Google has not revealed anything…revealing.
Fixed as suggested by http://qualapps.blogspot.com/2007/04/symbols-for-mfc-source-code.html:
Add directory “C:\Windows\Symbols\dll” to the beginning of the symbols list.
Clear symbols cache directory.