Danger states that their SDK is up at developer.danger.com--it's not. Hopefully it will be up soon.
The Danger SDK will allow third party developers to develop applications for the popular Hiptop device. If you haven't heard of the Hiptop that's probably because right now it is only being sold as the T-Mobile Sidekick. It's a small mobile device that focuses on the internet (browsing, email, IM) but is also a fully functional phone.
You can snag a Sidekick for $99 from Amazon.com.
Update: It's up. Start writing apps!