ARS Technica says the third beta of IE7 is slated for August:
Beta 2 went public in April, after Microsoft announced that the layout and rendering engine for IE 7 was "feature complete" and that no new changes, other than bug fixes, would be made outside of the user interface.
Now Microsoft is preparing a third beta, to be released some time in August. The extended beta period seems to have been prompted by compatibility issues that users have found with the first two IE 7 betas. One such issue, described in great detail on Microsoft's IEBlog, involved a problem where the browser would sometimes reload CAPTCHA images (character recognition tests used to fool spambots) when a network connection was idle..