Have you notice the presentation section in Dashboard of WordPress 2.3.1 does not include custom header image so I came up with my own solution. Check out my header at
http://amolpatil2k.x10hosting.com/blog/
I found that the default header image is present in wp-content/themes/default/images and is called kubrickheader.jpg
I downloaded this image and added my banner to it, then I uploaded this back. In Dashboard presentation tab - header image and color, click on revert and on toggle text display in Advanced. It worked for me. Maybe it may work for you guys too.
I know there must be better solutions but I didn't have the patience to search for them.