Just spent a fair bit of time trying to figure out why my site wasn't scaling properly (ie at ALL) on the iPhone. The culprit turned out to be the SimpleViewer plugin ( http://simpleviewer.net/simpleviewer/support/wp-simpleviewer/ ).
The author is aware of the issue and a fix is 'in the works' but not here yet.
Just wanted to let folks having this issue know and hopefully save some time and prevent too many iPhones from being thrown against the wall. (Mine wasn't but very nearly).