here is a tutorial of the opening process here:
Q: where I can get a proper screwdriver to open a Nintendo Gameboy?
search google for triwing screwdriver or try this set http://www.newelectronx.com/proddetail.php?prod=security-screwdriver-bit-set-100
or, alternatively, you might try to file down one wing of a philips head screwdriver ...
I think it's better to use a nail as the starting point for a Nintendo screwdriver; the metal is soft enough to be filed manually, and a dremmel tool would do wonders /Cycler
I actually have a few Gameboys that use just a phillips head screwdriver./Panda Jam
How Do I open my Gameboy Color? http://youtube.com/watch?v=lQikASmmpm8