I emailed Continental and got the following response:

"Miles currently have no expiration date; however, Continental Airlines reserves the right to impose expiration limits or terminate the OnePass program, thus terminating your ability to claim rewards. If you do not have activity in your OnePass account for a period of 18 months, Continental may close your account in accordance with the terms of the OnePass Member's Guide.

This e-mail inquiry is actually considered activity therefore, your OnePass miles will remain valid for at least another 18 months from today without any activity.

OnePass Service Center"

So, it looks like I am safe. :-)