Updated, I think Alaska changed the rules on their promo from $15 minimum to $25 minimum (or maybe I had it wrong before?).
Alaska Airlines
5/1/07 - 7/31/07
Only if you did not have an online ID prior to 4/20/07
250 miles per dine of $25 or more
3,000 miles maximum
Delta Airlines
5/1/07-6/30/07
Triple miles for Sunday-Thursday dines
3,000 miles maximum
Midwest Airlines, United Airlines, and US Airways
5/1/07 - 6/30/07
250 miles per dine of $25 or more
5,000 miles maximum
Northwest Airlines
4/1/07 - 6/30/07
Only if you did not have an online ID prior to 4/1/07
250 miles per dine of $25 or more
3,000 miles maximum
Continental Airlines
5/1/07 - 12/31/07
500 bonus miles for creating an online ID then having a dine of $20 or more within 30 days.
Priority Club
4/20/07 - 12/31/07
1,500 bonus points for creating an online ID then having a dine of $15 or more within 30 days.
Link to Bonus Offer (It doesn't seem to show up on the bonus page otherwise)
The Priority Club is more what I'd classify as a sign-up bonus and is basically only one dine, so anyone who wants to do that, do it whenver is convenient (maybe wait and see if there's a gap in promos at some point in the year).
By month, the limited time (not counting CO and PC that run until 12/31) promos currently known are:
June - Alaska, Delta, Midwest, Northwest, United, US Airways
July - Alaska