The so-called 3DK (3D Developer Kit) contains the two STP files you need (one for the actual shell, and one for the buttons that go in the shell), and the MakerBot team have helped Nokia with some practical tweaks to the plans to help printing.
From the wiki:
If you just want to start experimenting, use the first set in the list of our files, the STL files with components separated. STL files are the ones you can slice directly and print, and the first set separates out the buttons so you can choose not to print them. (You can always use the buttons that come with your phone.) You can also use the STL files as a starting point for modifications with some 3D design programs.
We've also provided STP files because they're a very common interchange format for 3D models in mechanical engineering. If you're really in to 3D mechanical design, you'll probably want to start with that set.
More details can be found here.