Nintendo and Sony are both holding their own pre-Tokyo Game Show press conferences. Nintendo’s event for their 3DS reveals is on September 13th at noon, Japan time. You can watch it streaming live as it happens. Check this page on the day of the event to find the Ustream (or NicoNico) link.
Sony’s is a day later on the 14th, at 1 PM, Japan time. That event can be watched live, too. You can catch that by visiting this page.