From what I see, Thermo is a tool that helps to integrate the visual design process in more tightly with creating Flash/Flex-based Rich Internet Applications. If you take a look at the demo, you will see how it can import artwork and use that as a template for creating a Flex-based application. From what I see of this application, this is really what the visual design feature of Flex Builder really should have been, and this also looks to be very suitable for quick semi-functional prototypes. The workflow looks like it will work well as well: Visual designers can use Thermo to start off the application, then Web developers can work with the same project in Flex Builder to add in the structure, additional functionality, and server integration.
Since Thermo creates code that can be also edited in Flex Builder, one thing I would like to see is that they keep Cairngorm in mind. That would help make Thermo more suitable for larger scale RIAs.