Well, that was quick. A day after the release of LostWinds on WiiWare, Frontier has confirmed on their website that they are working on a sequel. Here's how the site reads:
Click here to REGISTER FOR ADVANCE INFORMATION on Frontier’s forthcoming LostWinds sequel containing amazing new gameplay as Toku and Enril’s incredible adventure to save Mistralis continues - Frontier is registered under the UK’s Data Protection Act and won’t use your details for any other reason.
Gonintendo.com: LostWinds sequel confirmed!