American (
here):
Earn the actual base miles flown on American Airlines, American Eagle® and AmericanConnection® when traveling on a published fare ticket.
...
If you are an AAdvantage Executive Platinum®, AAdvantage Platinum®, or AAdvantage Gold® member, you will earn a minimum of 500 miles on applicable routes, except for codeshare services within Hawaii and with rail service operators booked as the AA flight number, which earn a minimum of 250 miles on eligible fares.
Continental (
here):
Effective for travel on or after Jan. 1, 2009, OnePass members will earn the actual miles flown rather than a minimum of 500 miles per flight for tickets purchased on or after Nov. 15, 2008. OnePass Elite members are exempt from this change.
U N I T E D (
here):
New for 2009: minimum flight mileage accrual for elites
For all United segments of less than 500 miles flown on or after January 1, 2009, elite members will earn a minimum of 500 redeemable miles and 500 Elite Qualifying Miles. Flight segments greater than 500 miles in length are not affected by this change. Complete information.
General members will continue to earn actual flight miles on all United flights.
U·S AIRWAYS (
here):
# Dividend Miles members traveling on US Airways will accrue based on the actual miles flown. Base members flying US Airways Shuttle will accrue the actual miles flown or 500 miles per segment, whichever is greater.
# Preferred members traveling on US Airways will accrue the actual miles flown or 500 miles per segment, whichever is greater.
So of the big domestic carriers (legacies I mean, we all know WN is big) DL is the only holdout for 500-mile minimums for all its general members.