Capcom US said information on a possible Street Fighter IV update was coming "mebbe Monday". Looks like they were a little off.
A teaser website has appeared on Capcom's Japanese portal, with the site's URL seemingly promising a "new warrior" while the site itself gives the date 2009.09.29 17:00 JST, presumably for said warrior's reveal. That's Tuesday, September 29, at 5pm Japan time, if you need to set a clock.