OS X 10.5.8, Safari 5.0.6, Flash Player 10.1.102.64. Happens with every YouTube link, though it only started within the last week or so. The preview image does have the play button.
Based on a few test clips I can confirm this with the same config on a 20" G5 'iSight' iMac. Fortunately (I seem to be a tad more visually inclined despite my voracious reading habit ), the direct link to the Youtube clip present next to the embedded clip window bypasses the Flash update requirement.