... it wouldn't do a Safe Boot (ie, no label identifying Safe Boot came up upon login, even though an unusual progress bar showed up beneath the Apple logo and spinning wheel during restart).
Given this description of events, your Mac may actually have booted into Safe Mode despite the lack of label to that effect. Check out the Apple Discussions thread
I don't think Mac OS X Lion is entering Safe Boot mode where behavior is seen similar to your iMac's.
It is interesting to note that several KB articles about
Safe Mode do not specifically mention Lion, but imply that they refer to it as well (see
Mac OS X: What is Safe Boot, Safe Mode? and
Mac OS X: Starting up in Safe Mode).
