This was taken directly from the WorldPerks T&C on their website.

3. WorldPerks miles have no expiration date. However, consistent with the general terms and conditions of the WorldPerks program, Northwest Airlines reserves the right to change the WorldPerks program at any time without notice, including imposition of expiration limits or reactivation fees. If no mileage from any source is posted to a WorldPerks member's account for three consecutive years, the account is subject to termination, including forfeiture of all accrued mileage.

I do see the last statement regarding the "3 years". The last earning of WorldPerks I had was on my flights in February 2002. I did fly on NW last October 2004, but that was on a CO award ticket - thus I did not earn either CO or NW mileage. Does this mean my 62K+ WorldPerks will disappear at the end of 2005?