SkankPhone is a part of Apple's internal debug utilities suite. It can be defined as a SpringBoard alternative. Instead of dividing the iPhone OS functions such as Phone, Safari etc. into standalone applications, Apple developers combined many functionalities into one app. SkankPhone has a integrated phone, web browser, music player and SMS editor. Pressing Home Button reveals a toolbar with 5 shortcuts to other SP sections: Phone, Web, SMS, Music and Setup. In Alpine_1A420 prototype firmware it also had a built-in Operator screen.
SkankPhone can coexist with SpringBoard. When the SkankPhone.app is linked into /Applications/SkankPhone.app, it shows up in SB and can be launched (if you have 1.x firmware). During launch, SkankPhone kills SpringBoard. Interestingly enough, this also applies to Alpine_1A420 which dates back to December 2006 and doesn't contain SpringBoard.