American: Counts your lifetime program miles (not just actual flown miles). One million miles gives you lifetime Gold status; two million (one million after the first million) gets you lifetime Platinum.

Continental: No such program currently, as far as I know. Members who maintained Gold Elite status for a certain number of years (6?) up to a certain point were given Gold Elite status for life. This was during CO's darkest era before the dramatic turnaround. This is now lifetime Platinum Elite (as Plat is now the highest level; gold used to be when the program started). Upgrades are also given as a gift; not sure how this works (what type).

TWA: Million milers (actual flight miles only) get Lifetime Platinum status (top-tier!), which is the most generous really. Not sure what will happen to this in the AA program.

Delta: Million milers (used to count all miles; now only flight miles count) get lifetime Silver Medallion status, gift(s), and stationery.

US Airways: No program that I know of but apparently some Million Miler tags were given out as a gift a while back.

United: One million actual flight miles gets liftime Premier Executive status and some upgrades as a gift.