This forum is mostly populated by people doing Apple approvable SDK apps. There are other forums which seem more popular among people writing stuff apps only for jailbroken devices (google for them).
You might be able link a button to another app (in a separate bundle) by having that app register a URL.
duny, check out the documentation on how to launch an application using the application URL scheme. if the app that you're trying to launch responds to an URL scheme, or is an app that you developed, you're in luck.