As I understand it (I don't have version 9, I run with 8.0.1.4+) version nine comes with a contact.aspx page for the contact form as opposed to using a topic as version 8+ does. I modified my topic to be a contactus.aspx page with codebehind - but realized afterward that I've got no idea how close it may or may not be to version 9's contact page.

In any case, mine (nor the topic) had an autoresponse ability for the holidays which were quickly approaching, so I whipped it up and thought I'd share just how easy it is:

First thing we have to do is set up two AppConfigs. One to determine if we're on vacation or not, and one to hold the message we want everyone to see if we are. Go in the admin to Configuration->Advanced->AppConfig Parameters. Add one named "Email.AutoRespond" with a value of "false"; and add another one "Email.AutoResponse" with a value of your holiday message or HTML based email response. Mine simply says "I'm out until December 28th." for testing purposes.

Next thing we do is determine what your version does for contacting. In version 8 by default you get the contact topic that forwards information to sendform.aspx for emailing. You'll want to open up the sendform.aspx.vb or sendform.aspx.cs codebehind, and navigate on down to the bottom. If you're on version nine, you can find the mail being sent in the contact.aspx codebehind. Whichever your case, find the line where AppLogic.SendMail is being called as this is where the actual email gets sent to you through the contact form.

You'll need a variable:
Code:
string autoResponse = AppLogic.AppConfig("Email.AutoResponse").Trim();
and an if clause after the send:
Code:
if (AppLogic.AppConfigBool("Email.AutoRespond")) {
}
Now you just call SendMail a second time, to write back the sender and say "HEY! AutoResponse Message here!" The variables you need to send are pretty much the same, except you have to flop the SendTo and SendFrom:
Code:
AppLogic.SendMail("Holiday Auto Response", autoResponse, true, SendTo, SendTo, SendFrom, SendName, "", AppLogic.MailServer());
And voila. But wait, there's more! If you act now I'll also throw in your first bit of troubleshooting at no extra charge!
It's not working. I did everything you said and I'm not getting an auto response although the emails are coming in.
Answer:
Did you turn your Email.AutoRespond appconfig that you just made to "true"? Don't forget to toggle it back to false when you're back in the office!
Have a happy holidays.