if you press F12 to check the markup, you'll find the following two issues:

   HTML1504: Unexpected end tag.

   File: Default.aspx, Line: 315, Column: 116

   HTML1508: Unmatched end tag.

   File: Default.aspx, Line: 808, Column: 1

This bad markup is causing some javascript errors - if you look at the page in firefox, you'll see that it posts and quits better than IE (still doesn't do a full postback) - if you use firebug you can see why (it's probable that the bad markup is causing this issue)

TypeError: dnn.controls is undefined

http://sharontest.the-dev-server.com/...

Line 15

You can verify this easily enough by changing to a default skin or removing other content, and then hunting down the bad markup