An update to an old post, I know, but I recently came across an issue that my previous fix didn't address.
If you need to use CSS3 on a site, putting the metatag in the master page defeats that support for IE9, so what I did was add this to the master page head (an asp:literal)
Then in the codebehind, I added this to the master page load event:
sender, EventArgs e)
Literal headLiteral =
"<meta http-equiv=\"X-UA-Compatible\" content=\"IE=EmulateIE7\" />"
On all my projects, I have a common Utility class in the App_Code directory, with some common static methods, such as tests for whether a page is in Admin, Edit or Preview mode.
Anyway, the idea is this injects the X-UA-Compatible meta tag in the head controls (at a specific location, which is why I can't simply add it to the controls collection) whenever a page is in Edit mode.
Until I upgrade this client to SF 4.0, this works for me, so I thought I'd share it.