Sign up for a new Starwood account using this link and enter your Northwest frequent flyer number and email address. Your account will start off with Gold status.
One caveat: the promotion specifies that the new account be designated for ‘airline direct deposit’ so any Starwood points accrued from hotel stays would be immediately sent over to your Northwest Airlines frequent flyer account rather than your Starwood account — but if you don’t want this option, it should be simple enough to call Starwood and ask them to end airline direct deposit.
The offer also specifies that it is for Northwest Airlines elite members but in practice it should not matter.
Update: This offer appears to be targeted and may not work for everyone.