I believe that the bonus is PER TICKET, NOT RT. Read carefully. It appears that if you book it as two separate one way Y tickets you will get 2-5,000 mile bonuses.
============================================
Got this today from Continental today--
To thank you for flying with us we are offering you bonus OnePass miles for
roundtrip travel to and from any destination between September 20, 2000 and
November 15, 2000.

Purchase a First Class or full-fare Y ticket and receive 5,000 OnePass miles
or receive 2,000 OnePass miles when you purchase any other coach fare
(excluding Q & T) for Coach Class travel. And, you'll earn 1,000 additional
OnePass miles for every E-Ticket you purchase on continental.com. It's our
way of thanking you for trusting us to make your flight as enjoyable as
possible.

TO SIGN UP FOR THIS OFFER: Call 1-800-346-6090 and use code 40002 to
register your OnePass account. Please have your OnePass account number