I phoned and was told the same as Lord Garlic.
If you enrolled pre 20th Sept and received the postcard original T&Cs apply.
If you enrolled pre 20th and were not targetted (even if you did not receive the postcard you may have been on the target list) the cap will apply.
Pre 20th enrolment cardholders will be notified by Amex of the change to their T&Cs.
My reading of the T&C's would suggest that anyone enrolled pre 20th Sept will receive unlimited bonus points as per the T&C's link that was posted.
http://www.americanexpress.com/australia/campaigns/bonuspoints/bonuspoints_faq.shtml
The amended Terms and Conditions, including the maximum of 10,000 bonus points per month, apply to all Cardmembers who enrolled in the offer from Saturday, 20 September.