Well, the 1.41 release seems to fix the "Not Implemented" error. It is available for both Jaguar and Panther.
However, now we have a new problem. Since Microsoft has decided to start charging for WebDav access to the Hotmail servers, unless people cough up, they will not be able to connect using programs like Outlook or Entourage. Unfortunately this also affects the plugin :(
There isn't a lot I can do about this, but I can verify that the plugin still works with "upgraded" Hotmail accounts