http://msdn2.microsoft.com/en-us/library/cc288325(VS.85).aspx
Whenever you need to define compatibility on a page you are most likely going to be defining it for IE7. Therefore, you will be using the following statement:
<meta http-equiv="X-UA-Compatible" content="IE=7" />
One thing that I have noticed, though, is that you need to make sure that this appears before other meta tags. It is ok if it is placed after the <title> element in the head, but it really should be before all other elements in the head. This is just an oddity that I have noticed with IE8 Beta 1.