Ryan Grieve used the Raspberry Pi mini-computer to make a functioning Pip-Boy 3000 and has documented the process of creating the Raspberry Pip-Boy on his blog.
"The idea was simple enough," he writes,"take a Raspberry Pi, write some visually interesting Python, stick it, a battery and a screen on my arm and hey presto. The execution was obviously not as straight forward, but I was committed."
With help from "a random internet stranger" who tweaked the display's visual style, he used an "appallingly cheap" LCD screen and map data from Open Street Map to create the interface.
You can check out a series of photos documenting his progress on his blog. If you're thinking about making your own Raspberry Pip-Boy, he's also made the code and images that power the device available online here.