Bob Eberlein and I spent quite a bit of time refining the spec last fall. I have been remiss in not posting the new version, but I expected it to change further. It hasn't, so I will now post what we arrived at a year ago. [Unfortunately, the forum is not letting me post it.]
The spec is not yet complete, for example, level 3 is still not specified, but it is a starting point. There will be some changes over the next few months as we start to define and implement level 3.
isee systems has been adding level 1 and level 2 XMILE support to iThink/STELLA and will release it with version 10. Bob was working on the same for Vensim, but, as you probably know, he has just left Ventana. I do not want to speak for Ventana, but it is fairly certain support for XMILE in Vensim will be delayed.
Bob had talked about setting up a separate forum for this topic, but for now, this appears to be it.
The STELLA/iThink binary format is not public. I do not think I would call it a secret, but we do not have time to document it. There isn't really much internal documentation either - other than the code that reads and writes it.