Look at this forum in IE9 (yeah I hate IE too, but lots of people are stuck with it). Notice the broken "compatibility" icon next to the search magnifying icon in the location toolbar? Want to get rid of it and ensure that IE is renedering your pages as you intended? Modify your templates to add the following IMMEDIATELY after the <head> tag:
<meta http-equiv="X-UA-Compatible" content="IE=edge" />
This meta tag must be the first item after the <head> tag. I wish the default SM templates would utilize this tag (although I understand why some people would prefer not)
Yep.
Some other settings:
http://www.simplemachines.org/community/index.php?topic=486798.msg3409015#msg3409015
http://www.simplemachines.org/community/index.php?topic=484678.msg3393669#msg3393669