You've got lots of answers. And those who said yes are correct, maybe.
1) Yes, it's possible.
2) It requires a court order (as some before me have already said) and your ISP.
3) The court is going to want a REALLY good reason - such as a threat of bodily harm.
Now...
Did you do it from your home computer? Then, yes - it is possible using your IP address and MAC address (the address inside your Network Interface Card which is specific to one and only one card). Your IP address CAN change every time you dial-in, or reset your high speed MODEM (notice I CAN - you COULD have a static IP address with high speed Internet).
THERE IS a way which makes it nearly impossible for it to be traced back to you - but I seriously doubt you used this method. It's called an anonymous relay server (or "remailer"). Basically, you send your message to a relay server with special commands at the top of your message (the commands vary depending on what type of relay you're using). The relay server doesn't record your original message, it forwards it according to the commands. Hence, the message now contains the IP/MAC address of the relay server, and not yours.
Most people who use relay servers use multiple sets of commands to route their message from one server, to another, to another (repeat any number of times - depending on how paranoid you are).
Here's some info on anonymous relay servers (too late for your case, though!):
http://email.about.com/od/anonymousemail/qt/How_to_Send_an_Anonymous_Email.htm
http://en.wikipedia.org/wiki/Cypherpunk
http://www.iusmentis.com/technology/remailers/nym.html
http://www.debian-administration.org/articles/483
That should be enough to keep you busy for a while. BE CAREFUL with ANY site that wants you to download software - you shouldn't NEED software other than your normal Email client (even Yahoo or gmail)!!! And, BE CAREFUL with web sites that want you to enter your message on THEIR system!!!
Good luck!