Flash Player 10.3 introduces auto-update notifications for Mac OS X. If you’d like to disable those for your managed clients (because you are distributing these updates via munki, for example), install this package:
This package installs a file at /Library/Application Support/Macromedia/mms.cfg with the following contents:
More info here:
Thanks to Patrick Fergus for the tip.
Update 02 July 2012: updated the link for the package.