Full story - Hope this clarifies some questions.
United Arlines will erase frequent-flier miles in accounts that have been inactive for 18 months as of the end of this year. United Mileage Plus miles previously expired after three years of no activity. The change will affect travelers who haven't flown on United or one of its 28 partners, redeemed miles or used Mileage Plus Visa credit cards since July 1, 2006. The airline said one effect of the change will be that its most loyal customers will compete with fewer people for award seats. US airways also closes accounts that have been inactive for 18 months. Continental and Northwest miles dont expire. American has a 36 month time limit. Delta has a 24 month limit on exisiting accounts, but new accounts are terminated if inactive after one year.
Sorry was lazy and only put in a couple of lines for original post.