It appears you need to fiddle with the registry as per this article: –
http://blogs.msdn.com/ie/archive/2009/03/10/more-ie8-extensibility-improvements.aspx
To run a WebBrowser control in IE8 Standards Mode, use the following new value into the registry:
[(HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE)\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION]
"MyApplication.exe" = dword 8000 (Hex: 0x1F40)
To run in IE7 Standards Mode, use the following registry value:
[(HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE)\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION]
"MyApplication.exe" = dword 7000 (Hex: 0x1B58)
For IE8 RTM, we’ve added a new “forced” IE8 Standards Mode value. When an application opts into this mode, the Web Browser control will use the IE8 User-Agent string and Browser Emulation mode strictly. It will also ignore fallback features such as the built-in Compatibility View list and the user-generated Compatibility View list when loading pages. To run in “forced” IE8 Standards Mode, use the following registry value:
[(HKEY_CURRENT_USER or HKEY_LOCAL_MACHINE)\Software\Microsoft\Internet Explorer\Main\FeatureControl\FEATURE_BROWSER_EMULATION]
"MyApplication.exe" = dword 8888 (Hex: 0x22B8)
In all of these examples, “MyApplication.exe” refers to the name of your application.