Strange. I found in my personal website the Child menus were not showing when I hovered over the root menu item.
There is nothing special about the menus. They are standard asp:menu items created when I dragged the menu tool from the navigation tools in Visual Studio 2008.
Has the hotfix to apply to your server.
In my case my site was hosted by another company and I did not have access to update asp.net on the server, so I chose workaround #2.
I found two workarounds:
1. Add the IE7 compatability Meta information: IE=EmulateIE7
2. Add this style:
and then apply it to the menu:
Let me know if this helped you!