Just curious has VMware support come back with anything?
I am running into a similar issue we have 100 SRM LUNs that need to be detach and it takes anywhere from 30 second to 10 minutes per detach to complete. SRM clean up eventually fails after 5 hours and I see the same results when I attempt to do a SRM force clean up.
One odd thing I observed is when you manually detach a LUN it sits at "THe deivce does not contain a diagnotic partaion" for 2-5 mins before it compeltes the detach.