I was having some problems with jQuery 1.4 in IE8, and after much debugging, found out what was causing it (at least in the sense that I figured out how to make it stop).  

Any page that included a call to $.post() or $.get() gave the error "Object doesn't support this property or method" on line 4792 of the unminified jquery file, which reads:

1. return new window.XMLHttpRequest();

I found out that the problem went away when I moved this meta tag

1. <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />

from below my script tags to above them.

I don't know how commonly people will run into this problem, but just thought I'd point it out.