The more I use my MB Air, the more I think that it was actually designed for the newer Penryn and upcoming Mobile Penryn Intel chips rather than the older (higher wattage) chips stuffed into the smaller form factor.

The heat buildup during movie playback has caused my computer to overheat more than a couple times. It shuts down one of the cores and causes the movie to playback very badly. It could also be that the GMA x3100 lacks any hardware decode for DIVX/XVID/h.264 video.

Even during normal browsing, design, HTML/CSS work, the fan comes on full blast and sometimes a core shuts down as well. 20 minutes of Youtube will almost always cause it to happen.

Reading the reviews of the new Macbook Pros with the Penryn chips, it looks like they run much cooler and the fans turn on much less often. I'm convinced that Apple went ahead with the Macbook Air even though there would be heat problems without the new processors.

I think there will be an update as early as June when Intel announces the Centrino 2 or Mobile Penryn / Montevina processors.