Thanks AppleMan. Your suggestion helped resolve an oddity with iMovie that had been troubling me since I posted my reply to Velanche.
When responding to Velanche's question, I firstly tested scrubbing through a recent Event in iMovie to double check what showed in the black box connected to the playhead. I was surprised to see that just the creation date and time was visible for each of the clips. The time did not change as I scrubbed through a clip. Each clip showed the same behaviour. From my past experience I thought that the time should, in fact, change as the playhead moved through the clip - just as you explained. I concluded that perhaps an update to iMovie had removed this feature, but felt somewhat puzzled.
It turns out that the Event I was testing comprised clips I had imported from Final Cut Pro X in the ProRes 422 format. Interestingly, ProRes 422 clips have a slightly different naming convention to normal iMovie clips that have been imported as Apple Intermediate Codec (AIC format). The word "clip" doesn't appear before the date and time. Also, "(id)" appears after the date and time, immediately before the extension .mov (for example: 2012-12-08 18_39_36 (id).mov).
It appears that, when scrubbing through ProRes 422 clips in an iMovie Event, the time does not update as the playhead moves - unlike AIC clips in iMovie Events. Of course, in Final Cut Pro X the elapsed time for each ProRes 422 clip is shown in the timecode viewer box in the centre toolbar.
So that explains things! Thought I was going crazy for a while! I had forgotten that the clips I was testing for Velanche were in ProRes 422 format - that's the first and only time I had imported this format to iMovie. I recall now that I was originally intending to edit the Project in FCP X, but then decided to use iMovie. Rather than re-importing from the camera or camera archive as AIC, I used the ProRes 422 clips already imported to FCP X.
Thanks again for your input AppleMan. I feel much happier now that I have an answer to why I wasn't seeing the time updating when scrubbing in an iMovie Event.
John