well - actually no - it isn't based on any sort of windows specific code. I assume you are referring to teh fact that Macs can't use hushmail. hush is written in java and Mac has been as usual absolute dipshits and not followed the java movement correctly - this will be slightly elliviated once os x is all done b/c the nice people at sun have decided to help them out (fucking idiots).
but no, it is not os specific in the code (www.hush.ai if you want to read through the code)
Where did you hear that the message line was not encrypted. It is not that I don't believe you, I sent a few test hushmail messages with "Can you read this?" in the message header and "Wow you can read my email" in the body. I ran a packet decoder against this conversation and could not read any of the information - message or body. I could see that they use Blowfish/SHA-1 1024 bit encryption for the key exchange but I could not read anything about the email. I also ran this same packet decoder on a message sent from my Yahoo account and could read everything as you will see from the attached images.
I think it is always good to use a non-descriptive message line anyway.