Hi Peter,
I see no difference in this behavior vs .NET 3.5.
www.mojoportal.com is currently hosted under .NET 3.5 and it also renders a style block
demo.mojoportal.com is hosted under .NET 4 and it also renders a style block.
In both cases it does not interfere with the menu style of included skins as far as I can tell. Maybe you have some extra junk in your App_Themes folder that makes it worse.
IncludeStyleBlock is a new property on the menu in .NET 4. Once we start shipping releases that are actually compiled for .NET 4 then we can set that to false and maybe it will not render this style block. The current release is compiled on 3.5 .NET so we have not yet tried using this property.
Best,
Joe