*collapses in a relieved heap on the floor*
Just wanted to say THANK YOU for this solution, esmi.
Not realising that Twenty Eleven was responsive layout I spent 100+ hours on getting the site looking perfect in every browser, but didn’t check it on the iPhone (I didn’t have one at the time. I do now). Then realised it was looking horrible on mobile devices because I’d changed the layout from fluid to fixed-width in the CSS.
Removing that code in the header has magically fixed the problem. Phew. I’ll pay more attention next time!