Ok, I looked at the code.  I am guessing you have a doctype that is still rendering in quirks mode for IE.   What doctype are you using?  

Regardless try switching to this (see this blog for how if needed).

<SkinDocType><![CDATA[<!DOCTYPE XHTML 1.0 Strict//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-strict.dtd">]]></SkinDocType>

The toolkit apparently does not support quirksmode.  The actual issue (for me) was that it could not calculate the screen width correctly.  We could hack the common.js file to work with quirksmode, but I'm sure something else would break...

John,

Would it be a legetimate enhancement request to have the skin parser generate these DOCTYPEs? That would definately be helpful and since parsing skins is on a skin-by-skin basis, the host could do it at their discretion.

Thoughts?

I must be doing something wrong. I created a file called Horizontal Menu - Fixed Width.doctype.xml. Then I applied the Horizontal Menu - Fixed Width.ascx skin to the site, the modalpopup extender doesn't work. I've applied the same skin to the page itself and the result is the same. I get a page with an error.

Tried clearing the cache and also reparsing the default dnn skin. How did you get it to work?