I don't know whether there's a sweet spot feel in play (I don't play 4E), but I definitely don't see one based on levels of published adventures. I charted them on a Power Point slide, and it shows a heavily weighted bias towards low level adventures, with no sweet spot. For a new game, this would sound logical to me. As time goes on, and there are more higher level adventures, it should even out. But, there's definitely not a concentration of adventures in the 8th to 12th range.
I included 2009 releases on the chart (which are mostly higher level adventures) along with all other WoTC officialy published adventures. The chart in the right hand corner of the picture is starting level distribution.