I may have some bad news, as changes are evident in the statements.
For a Platinum Charge Card, statement issued 5 April 2011 (before the change kicked in), simply had "please ensure your payment reaches us by 19/04/11 to avoid...". With Easter and some slow bank BPay (plus paying it later anyway), payment was received by Amex on 28/04/2011 (but according to BPay regulations, would / should have been treated as being made earlier than that).
No late charges or anything have appeared on the next statement.
Statement issued 04 May 2011, and the wording etc has changed.
Under the summary boxes of charges and payments, is an ominous simple 2 lines
Due Date
May 18, 2011
The message below that has also changed to:
Thank you for using our services. Please ensure your payment reaches us by 18/5/2011 or additional charges may apply.
I am inclined to think this indicates (as previously posted) a change in the system. Particularly ominous is the use of "Due Date" which has never before appeared on the Charge Card statement - now triggering the ability to apply the changed T&Cs.
There is probably a grace period (also previously posted) of 7 days. This is normally included in credit card systems as it is easier than trying to adjust for things like BPay rules, internal glitches where processing runs a day late etc - easier just to give a 7 day grace, but be black and white after that and simply use the day the transaction is processed to the account.