Under Windows XP we use apr_shm_create() with APR v.1.2.11 to create a shared memory segment, but under Windows Vista those same calls return error 5 (access violation).  From researching the issue it appears to be a limitation that Vista intentionally imposes.  Only applications running as services get the privilege of being in "session 0", which seems to be the requirement for putting shared memory in the global namespace.
The MSDN documentation does say that if you have the "Create Global" privilege, you should be able to create it even if you are not in session 0, so maybe this is just a bug in Vista.  I don't know.
Do you guys have plans for a fix or work-around for APR?
Here's a link to an MSDN forum post about the issue:
MSDN info on CreateFileMapping():