Have you taken a look at the man page for “mail?”. (Note: this is the unix mail command, not the application “Mail” that we are used to using). In looking it over, I don’t see a provision for sending HTML formatted mail.
That being said, the scripting dictionary for Mail (the Apple-provided GUI mail program, otherwise known as “Mail.app”) does have a “headers” property, but in my version of Mail (1.2.5 that came with OS X 10.2.8) this is a read-only property and can’t be set during a “make new message” command.
If anyone has info about making HTML messages in mail or Mail, please post.