When you hover over the Log In, the credentials form does appear, but as soon as you move into the
Username or Password text-field, this form goes away. If you avoid the text fields you can move the mouse to the "Log in" button, "Remember Me" checkbox or "Register for a Sencha account" link. Clicking on the Log in button with empty credentials does give you the alternative login form, with the invalid username or password warning.
Using Chrome 9.0.597.94 on Windows Vista 64-bit, same result with Chrome 9.0.597.84 on Ubuntu 32-bit.
Just in case you are wondering, I used Safari to get here, before I figured out the avoid the text-field trick