I installed it soon after it was released and I just saw it pop up again in a Windows Update a few minutes ago. I didn't watch the update progress, but when I turned back to the SB a few minutes later it said that there had been a problem with the updates and there was a Retry button. I clicked Retry and it said I was up to date. Rebooting and rerunning Update still says I'm up to date. Strange.
Another thing I've noticed is there seems to always be a new Defender Definitions update when I run Update, unless I immediately "check for updates" right after it installs. Leave it for a few hours and there it is again. I suppose I should be glad if they really are updating it that often with new virus definitions, but it seems to appear much more frequently than any Win 8 machine I've maintained.