The full rules for the American Airlines 30 Days of Dining (May 18 - June 16, 2011) promo are contained
here, in this new thread.
(Moderator hat on: Please follow or add further discussion on this specific program in that thread; thanks!)
Good news - NO limit on bonus miles / times you can earn in thirty days.
Moderately bad news - though not disclosed on the registration page, you apparently must rate the dine within thirty days. (Moderately bad because it is not disclosed up front, and is revealed only as you pore through the registration confirmation e-mail.
)
As to the rest, a "month" seems to be a calendar month - e.g. January 1 - 31; the limits for dining range from one dine per month to $600 monthly (older restos) to $1,000 (newer ones) to a few unlimited. And I suppose any resto could create their own limits.
Also, don't forget some restaurants accept some credit cards they do not award points for, notably some restaurants that accept AMEX but to not allow mileage accrual. It's worth checking periodically for the places you patronize - I do, as my HH AMEX allows me to double dip and earn AA miles and HH points, but if the place accepts AMEX but doesn't give miles for it, I get a paltry 3 m HH points per dollar alone.