Over the weekend I pushed out a small but important update to my starter master pages. When I originally created the starter master pages I used both the Design Manager feature as well as Seattle and Oslo to decide the best place to put things in the master page. There was a discrepancy between where the Design Manager and the default master pages placed the SPWebPartManager control. Turns out the Design Manager puts this control in the <head> section which is incorrect, it should be in the <body>. Having the control load before the <body> caused a host of mysterious problems so I have moved it to the body and I urge everyone to follow suit.
UPDATE: 2010 Starter Master Pages can be found here: http://startermasterpages.codeplex.com/releases/view/41533