According to Kristin Dos Santos of E! Online, Supernatural panel at San Diego Comic-Con 2008 will be held on Sunday, July 27, 11:15 a.m.–12:15 p.m.
Please remember that this is only a tentative schedule. The complete programming schedule will be available approximately ten days to two weeks prior to the show.
Source: Watch with Kristin, E! Online, San Diego Comic-Con 2008 website.