Those two are XHTML transitional, Atahualpa is strict, it may be related to that
It's working on my test site, see http://wp27.bytesforall.com/
Change the content in the Facebook text widget to the correct version
(Note the added "type=...")
You also have a wrong <br> tag at the top of the twitter tag
should be <br />
If this doesn't help, try changing at the top of header.php
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Strict//EN"
<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
just to see if this would help. This should make IE less picky about other errors that may be on your site