Just matched from CO Plat to NWA Plat. The process was easy - I just sent in a cover letter, copy of my card and activity from co website. Response took one week. Got the email today:

"A variety of criteria, including information from your Continental
Airlines mileage summary, is considered when evaluating requests from
members to match elite status from another airline. After reviewing the
information provided to us on your OnePass account, your Northwest
WorldPerks account has been upgraded to Platinum Elite status. This
one-time complimentary membership upgrade will remain valid through the
2010 elite program year. "

Looks like the status wil extend through 2010. I did ask in the cover letter for the status to be through 2010 as I already qualified for 2010 Plat status with CO.