I worked at a company that wanted to use these as cheap and disposable PDAs. At the time the Cybiko company was collapsing but they kept promising that new hardware was on the way, presumably to keep us buying their existing stock.

IIRC the SDK consisted of a (limited) C compiler that produced bytecode that the Cybiko ran in a virtual machine. The VM was intended to future-proof the device in case they decided to switch CPU architectures somewhere down the line. A little over-engineered, given its eventual fate.

We got hold of the source code for the Cybiko's web browser because we wanted to do something similar. A PC ran a server component and did all of the hard work interacting with the internet and parsing HTML, while the Cybiko ran a client that connected to the PC over its RF hardware. In the end we didn't get very far - it was my first C project, the SDK wasn't great, and the browser source was written in Russian.

I still have two unopened Xtremes somewhere.