Unless something has changed in the last day or two, I believe Delta's new policy is TWO years, not one. USAir recently cut the expiration period to 18 months, but I think it's a rolling 18 months, which can be much more restrictive than merely "within a calendar 18-month period." I don't know all the policies of all the programs, but the tightest time period for points expiration I know about is either USAir's 18 months (if indeed it's a rolling period) or Starwood's requirement that the account be active every calendar year.

Interestingly, Starwood's 12 month policy has evidently been in place from the beginning, or at least for a long time, and it's very rarely the focus of complaints, whereas DL and USAir are under criticism because they've changed their policies to more restrictive ones.