Check the following explanation from here:
Sometimes the Compatibility View
button isn’t displayed. The button is
located on the address bar next to the
‘stop’ and ‘refresh’ buttons. There
are a few cases where there’s no
action for a user take and, thus, the
Compatibility View button will not
show:
If you're viewing an
internal-to-Internet Explorer page
(such as about:InPrivate)
If you're viewing a page that has
declared it's "ready" for Internet
Explorer 8 through use of the
versioning tag / HTTP header
(it doesn’t matter if this tag
triggers Quirks, IE7 Standards, or IE8
Standards, the button won’t be
displayed)
If you're viewing an intranet page and
you have the ‘Display intranet sites
in Compatibility View’ checkbox
selected
If you're viewing any webpage and you
have the ‘Display all websites in
Compatibility View’ checkbox selected
If you're viewing a webpage that is
included on the Microsoft-supplied
compatibility view updates list and
you have the ‘Include updated website
lists from Microsoft’ checkbox
selected
If you've toggled either the ‘Document
Mode’ or ‘Browser Mode’ settings via
the Developer Toolbar