On a side note to this I suffered the same problem a while back and through the post below asked for any solutions however none came forward.
http://forum.x-cart.com/showthread.php?t=37531
My solution was different to that listed here. In fact I still have the "float:left" listed in my CSS.
I found that one of my tpl files (I believe it was dialog.tpl but I could be mistaken because it's been a while and I've changed many things since) had a rouge navigation.tpl include statement. Whether it got there as a result of not follwoing the rounded box code exactly or by some other little gremlin I'm not sure but once removed the page layouts went back to normal.
So any one suffering the above problem that tries the "float:left" solution and doesn't fix the problem hunt for rouge naviagtion.tpl include statements