Is it possible to write an iPhone app to control the built in iPod?

Ah, sweet, thanks!

I checked out this page: http://developer.apple.com/library/ios/#documentation/Miscellaneous/Conceptual/iPhoneOSTechOverview/iPhoneOSFrameworks/iPhoneOSFrameworks.html

It isn't possible for me to intercept incoming calls, or access the complete functionality of the mail app, is there?

To intercept calls and mail you should access private APIs.

Hm.. by private APIs, do you mean you need to be in the developer program to use them? Or do we need to develop those ourselves?

If its the first, could you please point me to them? =)

These private APIs are available for all. You can check out http://iphonedevwiki.net for private frameworks. These private frameworks are often used for jailbroken devices development (the private framework's header you find in this site are reverse engineered by hackers). AppStore forbids developers from using these private APIs. If you use such APIs in your app, it will be rejected by AppStore.

-- There's also some apps in appstore which are exceptions to this rule (cisco's anyconnect, tapdictionary, etc...). Probably these applications have signed some extra agreements with apple regarding using these private frameworks. Ask Apple developer support for accurate information on this...
If you called them, inform us from what they told you.
You will be appreciated alot...

>>Ask Apple developer support for accurate information on this...

Alright, will do =), thanks for the information =D