Good feeling! I just implemented a welcome app and it's contact form submission in Facebook Apps. The implementation of the contact form is simple but it seems so complex when the regular post method does not work and I had to change the post method to get in php.
Wish these details were posted before somewhere, but it's not easy to find a solution when I was searching for a specific text.
The form actually resides on my server, but the URL is of apps.facebook.com, so the formmail.php script that, I used because of php, runs into problem, as somehow it's trying to process the form through apps.facebook.com domain rather than that of the domain inside the iframe of my server.
Any way, if anyone is working on a Facebook app or Facebook submission form and needs help, than post your question in the comments section below and I will help out in what ever form I can.
Happy Programming :)