Update! Looks like Apple claim of Mac Pro in stores today was BOGUS. So no info on memory limits will be coming today after all. Not very nice of Apple to send emails with false information. My Mac Pro still due for shipping on Aug 23rd.
Usually only base models show up the first day, so you’re likely to see only the 2.8GHz quad-core and/or 2.4GHz 8-core. But there is always the possibility of new precedent.
My local store says they "have nothing yet" when I called at 9am PST today.