In spaces that I follow, I get email notifications, since I ask for them. Jon Reed mentioned he gets some but not all, and once I checked, I see the same symptoms. As I usually delete these after scanning, it's not easy to verify I did not receive the notice in the first place.
I am tracking one in the Private SAP Mentor space, where I can see posts on the SCN site that did not all reach my inbox. There is a possibility the message was discarded by our corporate rules (where I cannot see it without asking the network goddesses), rather than my personal junk mail settings (where I could see it for a time).
If others have noticed similar patterns, perhaps setting up messages on test forums could isolate the cause?
Searching through prior posts I have not found this reported. E.g.:
http://scn.sap.com/thread/3361630
http://scn.sap.com/thread/3363547
http://scn.sap.com/thread/3275601