In many cases you really wouldn't care about the schematics for your cell phone; however, when designing and building complex systems it is a priceless advantage to have the design and the schematics of the individual components at your disposal. You can avoid many surprises during integration since you have a chance to find out exactly what you are buying instead of just getting a black box which may or may not live up to your expectations (lets face it, there always a huge gap between what a sales department advertises and what the technical department delivers). Moreover, you have the freedom to use the hardware you bought the way you want to and not the way the manufacturer wants you to. And you don't risk being sued just because you opened up the box to look at what's under the hood.
Hopefully, it will not take many years before we will see complete space systems on the list :-)
Daisy MP3 player - An open source MP3 player
The Neo FreeRunner open source cell phone.