Regarding Activation IDs:
Mystery #1: At first because I didn't know any better, I was only using the command slmgr /ato and that worked on a couple of Windows 2008 R2 servers. The servers showed as activated and got the March Rollup. I'm not sure why I didn't need an activation ID.
Mystery #2: I'm glad I read this article because I discovered that slmgr /ato needs to be followed by the activation ID. (Apparently not always....See Mystery #1.) So what makes this also a mystery is that my Windows 2008R2 activation IDs do not match what is in the table above. Once I did add my servers' activation ID (not what's in the table above) to the command on the problem servers, I finally got them activated. I've read through the comments on this page but nobody else seems to have found the same discrepancy.