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. Interesting thing is that it also applies to prototype Alpine_1A420 version which dates back to December 2006 and doesn't contain SpringBoard, suggesting that software engineers were already working on the SB, but it was in early stage of development so they didn't include it.