How would I go about updating the Planet shader to Unity 3?
Putting that shader into a Unity 3 project should cause an automated attempt to update it. If it doesn’t work automatically (and it probably won’t, given the nature of this shader), you have to follow the instructions given to you by the compiler warnings and errors. It might help to look at the #pragma debug output of a surface shader and compare it to the source of the equivalent Unity 2.6 shader to see how different constructs and macros have changed in Unity 3.