Be aware that United Airlines has changed its frequent flyer program.

Accounts which have NO mileage activity within 18 months will lose the mileage...
regardless of how many miles you have in the account.
(it used to be 36 months - now reduced to 18 months of no mileage activity)

See this message from United Airlines:

http://www.united.com/page/article/1,,52106,00.html .

Q. When will my miles expire?
A. As of December 31, 2007, miles will expire after 18 months of no activity on your account. For example, if there has been no activity since July 1, 2006, all miles in your Mileage Plus account will expire on December 31, 2007.

...and many more questions and answers about the change at United Airlines.

Years ago, I was a happy customer of United...but when they changed the rules - about 10 years ago - I was unaware of the change...
and I lost many miles. To say the least - United lost me as a customer and I have been an annual Platinum Elite frequent flyer with Northwest ever since.

TravelerJim