[UPDATE 6 December 2010] Sorry, It seems the solution in this post no longer apply. I don't think there is a way to send a message to an event attendees on Facebook.
A few months ago when I created an event through one of my fan pages on Facebook, I discovered that I could not send a message to the guests list. After some searching and consulting with friends I found the solution on Facebook's FAQ page:
As the admin of a fan page, you cannot message your guests. You have to appoint someone else to be the admin of the event. Then, he will be able to send the guests a message for you.
(I have no idea why that is, of course). To appoint someone else to be the admin of your event, simply look him up in the guests list and click of the 'Make Admin' button next to his name. Image by Spencer E Holtaway