I have seen it on three projects we do for a tv-show. Project format is 1080p25 originated from AVC-Intra 100. But we do lots of these and I only had it on three of them, on two different machines (a MacPro and an iMac27/i7 both running Lion).
The issue does not seem to be constant, it only happens when you zoom in on the timeline to a certain extent. On two projects I had it with a series of titles, on another project I saw it with a bunch of connected clips. When you zoom in on the timeline the clips don't show thumbnails but just a black box. Not all connected clips turn black, just some of them, and when you zoom the timeline to fit and then zoom in again sometimes the issue is gone, sometimes not.
Strangely enough I just loaded these three projects to post a screenshot and now I cannot reproduce the issue anymore. Go figure. So I guess it's an intermittent bug. I have asked our editors to make a screenshot if and when this happens again so I can send a more detailed report.