Drag the file to IDA, change CPU into 'ppc', then click 'ok' a few times, that's it. It won't disasm some instructions (Takires is working on a plugin for that) and it won't know the entrypoint, therefore you have to disasm instructions manually by pressing 'C'. Keep in mind that instructions on PPC are ALWAYS 4 bytes long, so you have to press 'C' on a 4th byte (i mean an offset ending with 0, 4, 8 or c). So scroll a bit through the file and try to find code.
Program entry point for the Cx sections is in the header, normally at offset 0x8 if I remember correctly
*EDIT* Ah, the plugin from Takires just got released, great work: http://www.xboxhacker.net/index.php?topic=7921.0