Delta, which has not had one of the more popular frequjent-flier programs in recent years, may have just made itself the belle of the ball. The airline says it's miles no longer expire after some period of time --18 months to 3 years has been standard at most carriers. Most programs over the last few years have been moving the other direction, making it harder to use miles after they're earned. The move suprised even veteran watchers of frequent-flier and other loyalty programs, including Tim Winship of the SmarterTravel.com Web site. His posting today, with lots of good background on recent developments in frequent-flier programs, can be found here.