Official Onepass Mile expiration statement:

According to the Continental Onepass Member Advisory page the following official conditions exist as of 12/2007: Miles currently have no expiry date; however, Continental Airlines reserves the right to impose expiry limits or terminate the OnePass program, thus terminating your ability to claim rewards.

Official caveat aka Inactive Membership:

Immediately below the "no expiry" date statement you can find the Inactive Membership section which states:

If no mileage is deposited in your account for 18 consecutive months, your membership may be cancelled, and your miles may be forfeited. To keep your account active, you can earn mileage with a variety of airline, non-travel and shopping partners. Please visit the Earn Miles section of this site for a full list of earnings partners.

How to keep your account active:

Any activity will reset your account's activity date. This does not just include flying and accumulating miles, but performing any of the partner activities as well. This includes but is not limited to hotel stays, car rentals, online purchases, and miles credited via a linked credit card. Third party mileage offers and incentive programs such as E-rewards/E-miles and other activities that deposit miles will also reset your activity date.

NOTE: The official statement lists deposits not withdrawals. Using miles does not count as activity officially.

NOTE: Mileage expiration is currently not enforced. However this may change at ANY time and you should plan accordingly.