I'm new to iPhone developement so please forgive me this very basic question. Since I can download the iPhone SDK for free I was wondering if it's possible to use ad-hoc distribution without joining any of the developer programs (standard or enterprise).
No. You must be a registered developer so you can create, download, and install development and distribution certificates. These are required to get an app on a real device.
The only way to get around all of this is to work with jailbroken devices.