Walker,
I filed a bug about styles if the control ID starts with an _ but that said it's very low priority because it has an obvious workaround and is somewhat nonstandard. However, you shouldn't be seeing this happen normally. It looks like there is something wrong with your control tree. Here's what I tried and what I got:
<%@ Master Language="VB" %>
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">
<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
<title>Untitled Page</title>
</head>
<body>
<form id="form1" runat="server">
<div>
<asp:Menu ID="Menu1" runat="server">
<StaticMenuItemStyle BackColor="#80FF80" />
<Items>
<asp:MenuItem Text="New Item" Value="New Item" />
</Items>
</asp:Menu>
<asp:contentplaceholder id="ContentPlaceHolder1" runat="server">
</asp:contentplaceholder>
</div>
</form>
</body>
</html>
Output:
<style type="text/css">
.ctl00_Menu1_0 { background-color:white;visibility:hidden;display:none;position:absolute;left:0px;top:0px; }
.ctl00_Menu1_1 { text-decoration:none; }
.ctl00_Menu1_2 { }
.ctl00_Menu1_3 { }
.ctl00_Menu1_4 { background-color:#80FF80; }
</style></head>
One thing you can do to debug is set Trace="true" in your <%@ Page %> directive of the content page. You'll see a lot of diagnostic infomration, look for the section called "Control Tree". You may be able to decipher from the output where the problematic control is. If you figure it out, please let me know what you find. If you are stuck, can you share some of your masterpage markup with me so I can try it on my side.
--
Danny
disclaimer: Information provided is 'as is' and conveys no warranties or guarantees.