I've always been told that if you mail a piece of mail from Main Street USA in the Magic Kingdom it will have a "Magic Kingdom" postmark on it... but each time I've mailed a postcard home it always just had "Orlando" on the card.
However, I found out this past week that you CAN get Main Street USA, Magic Kingdom marked on your mail! If you take your mail to City Hall and ask for the Main Street post mark, they will stamp your mail for you! Then when it goes in the mailbox, it will get the Orlando postmark at the post office.
So, next time you are at WDW, if you have a special card to send to someone (like this awesome Birthday card from my parents) visit City Hall first and have them stamp your envelope! I'll assume they will do this for postcards purchased in the park too!
Happy Mailing! :)