I've just been working through exactly the same thing and as usual the answer is simple, but hard to find - you need to have menualignment="right"
<dnn:MENU runat="server" id="dnnMENU" menualignment="right" />
Solpartmenu sets up it's menu in a table like this
<table><tr><td>menu1</td><td>menu2</td><td>menu3</td><td width=100%><img src="/spacer.gif"></td></tr></table>
so a float right won't work - the table is pushed to 100% of the width of the containing cell/div
I also tried containing the menu table in a div in my skin and floating that right, but that only works in some browsers.
When you set the menualignment="right" attribute, the menu table draws with the 'filler' cell at the left end
<table><tr><td width=100%><img src="/spacer.gif"></td><td>menu1</td><td>menu2</td><td>menu3</td></tr></table>
forcing the menu nodes to the right.
Enjoy