You guys got me worried.

I just called NW. It was changed in 1999 to no expiration.

I believe CO is the same (no expiration)

Look, this is just a semantics game played by NW and CO. Your miles do not expire, but your account can be closed and you forfeit your miles. This is not technically "expiration" but you lose your miles all the same. I already quoted the CO rules above. Here are the NW rules (also quoted by the_traveler above):
Try asking them about account closing due to inactivity and see what they say. If they still say no, point out the clause in the T&C and say, does this mean this is not valid? If they say yes, make sure you get it in writing!

Note this policy is actually worse than most other US airlines, but actually looks better on the surface. Most other US airlines only require mileage activity. NW and CO say they actually require mileage posting.

Miles don't expire, but an account with no activity over a period of time may be closed (thus forfeiting accumulated miles).

So it's confusing to say that miles don't expire, when you can lose all of your miles if you don't earn or burn.

The importance of the distinction is that when miles expire you need to use them within a period of time or those miles will be lost (but not other, more recently acquired miles).

I actually like the change, but it can be confusing to some members.

Note that 'expiring miles' (or actually credits) is what Southwest, JetBlue, and AirTran have ... use credits in 12 months or lose them. Note that you don't lose all your credits necessarily ... just those that are more than 12 months old.

Starwood has an interesting policy -- points don't expire (note that there's a wrinkle that deals with Starwood Vacation Ownership incentive points) but an account will be considered inactive and can be closed if there's no earning at a Starwood hotel or with a Starwood co-branded credit card within a 12 month period.

Can you elaborate on this I assumed Jaguar miles were good for 3 years. I have never used them but thought they were still good?

My BA miles expired recently (50K of them too). Thanks to some friendly advice from this site (and some plain old pleading), they were reinstated.

Or keep good records.

So your miles aren't "forfeited."

It's easy enough to keep accounts alive, which is the subject of other threads.

My general rule is if i haven't used a mileage program in a while, I try to credit it the next time I use a rental car.

usually the rental car miles aren't really very significant in number, but they ARE enough to show mileage activity in an account, which generally means the account stays open another three years

I use the same approach for the same reason.

Renting a car for a day... if not for the weekend is the best way to show some mileage activity on the account.