Hey TwoBitTxn,
You have likely not been hacked. The spammer probably just "spoofed" the headers in your email, meaning that their email software inserted your email address in botth the To: and ReplyTo fields of the email they sent, thereby ensuring that their own email would not be visible.
If you look in your email program, you should find the option to View Full Headers, which will show you the IP address of the server that the spammer either used or (more likely) borrowed, and maybe a domain name. And yes, these guys have no ethics.
You can report the violator to Yahoo if the source was a Yahoo account, or you can look for the domain from which the perpetrator sent the email and send a compaint to that box. You can use the whois.net to look up the domain name and find the hosting provider, and send a complaint to them. Just be sure it's an ISP and not a self-contained spammer...
Hope this helps,
John
PS: Read the Yahoo Help section on their Bulk Email box. You can use their software to filter spam.