We've been doing a lot of work with the TeamworkPM API for our project supervision SAAS AscentList (there's been a beta available for Basecamp before we switched to Teamwork but we're now ready to release for Teamwork). Sign up here. Here's a quick preview:
Here's a list of tips to help you get the most out of the TeamworkPM API:
Just like Basecamp (Classic)
The first cool thing about the TeamworkPM API is that it's a lot like Basecamp Classic's API which means developers with a good Basecamp helper app should be able to port their technology over relatively easily.