I received two postcards in the mail. One was received a month ago and gave the dates as 4/1/06-6/30/06. Th new card that arrived today just said charges made through 6/30/06 and in the fine print gave 4/1/06 as the starting date. Both said that you didn't have to register.
The funny thing is that when I entered my card number on the link provided above, it said that I wasn't eligible.
I called Amex and they verified that I was registered already.
Also please remember that this promotion is counted on what you charge over the first $1500 miles that you spend each month in this 3 month period. You
do not receive the bonus miles on the first $1500 spent each month.