> Is the issue with a length limit on data: uri's? something with the CSP?

The problem is if we left the WebAssembly API enabled as-is, then people could fetch code from the internet and execute it. We already disable JS eval() so we had to disable the WebAssembly API too.

> Will there be some kind of session storage or is it completly stateless?

We're working on storage. It's a complicated problem. :) (See elsewhere in this thread.)

In the meantime, for many use cases you can use the HTTP cache as a sort of storage. Also, each worker instance has global variables which are writable -- there's no guarantee we won't reset your worker at any time, but generally one worker instance will handle multiple requests so can do some in-memory caching in globals.