The solpartmenu has hardcoded borders for hovering.  This issue is possible work around with the latest version of the menu, or you can simply specify the borders to match your background color.  To upgrade your menu, grab the latest solpartwebcontrols.dll and spmenu.js files from either the latest dnn build or my site.  Copy the dll into your site's bin folder, and the spmenu.js into the controls\solpartmenu folder.  After updating if you are still having issues, please post a url for me to review.

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Sorry to bother you again Jon, but now there is a quirk where if I hover over a submenu item, the background and font color of the item changes as stated in the CSS, but when I hover over a main menu item, the background changes but the font color does not change.  The font color changes if I specify a background color instead of background image.  I'm assuming I'll have to wait for a new release for the solution to this.  To take an easier way out, would it be possible to disable the background image/color hovering attribute for the main menu item?  An example of this would be the DotNetNuke site...lets say when you hover over 'About', only the text color changes, and then for the submenu items, there actually is a red background upon hover of each item.  Don't know if this helps, but here is all the CSS for the solpartmenu:

.MainMenu_MenuContainer {
font-family: verdana;
color: #FFFFFF;
font-size: 10pt;
font-weight: bold;
text-align: left;
background-color: transparent;
}
.MainMenu_MenuBar {
font-family: verdana;
color: #FFFFFF;
font-size: 10pt;
font-weight: bold;
background-color: transparent;
font-weight: normal;
text-align: left;
}
.MainMenu_MenuItem {
font-family: verdana;
color: #FFFFFF;
font-size: 10pt;
font-weight: bold;
text-align: left;
padding-right: 0px;
padding-left: 0px;
padding-bottom: 0px;
padding-top: 0px; 
border-top-style: none;
border-right-style: none;
border-left-style: none;
border-bottom-style: none;
border-spacing: 0px;
height: 20px
}
.MainMenu_MenuIcon {
display: none;
}
.MainMenu_SubMenu {
background-color: #000033;
filter:progid:DXImageTransform.Microsoft.Shadow(color='DimGray', Direction=135, Strength=3);
}
.MainMenu_MenuBreak {
background-color: #000033;
}
.MainMenu_MenuItemSel {
background: #FFFFFF;
font-family: verdana;
color: #000033;
font-size: 10pt;
font-weight: bold;
text-align: left;
padding-right: 0px;
padding-left: 0px;
padding-bottom: 0px;
padding-top: 0px; 
border-top-style: none;
border-right-style: none;
border-left-style: none;
border-bottom-style: none;
border-spacing: 0px;
height: 20px
}
.MainMenu_MenuArrow {
padding-right: 0px;
padding-left: 0px;
padding-bottom: 0px;
padding-top: 0px; 
border-top-style: none;
border-right-style: none;
border-left-style: none;
border-bottom-style: none;
border-spacing: 0px;
}
.MainMenu_RootMenuArrow {
}
.MainMenu_MenuIcon
{
display: none;
}