Seems to happen on all versions of Safari (shipped with Leopard) to 528.7+. WebKit/Safari do not offer to save passwords for websites using strict XHTML 1.1 and correct Content-Type: application/xhtml+xml. Visit https://satdb.org/Login.do Type any username and password. Upon clicking submit, it is expected that Safari/WebKit will offer to save the password. It does not. If the served Content-type is changed to text/html, WebKit DOES offer to save passwords. It should save passwords regardless of the type of page.
Password autofill is part of the Safari application, not the WebKit framework. See <http://webkit.org/blog/101/back-to-basics/> for more information about the distinction. This bug report should be filed against Safari at <http://bugreport.apple.com/>. This bug will be closed as INVALID per our policy for handling bugs outside of WebKit.