1. My understanding is that Time Machine will not recognize a drive that is not GUID. Period.
2.
Extrapolating from
http://en.wikipedia.org/wiki/Applepartitionmap and some experiences of other users who were having trouble with Boot Camp because their drives were not formatted GUID, it sounds like if you wanted your external disk to be bootable for both your Intel iMac
and your PPC iBook and mini, then you would want the Apple Partition Map. I won't claim that my extrapolation is accurate, but it's derived from the idea that an intel mac can boot from an APM partition, but I don't know if the PPC will boot from GUID. So because you don't intend to use the drive as a startup disk, you can use GUID anyway. 🙂
2.5 See
http://en.wikipedia.org/wiki/GUIDPartitionTable GUID is for Intel machines and is an improvement over MBR (Master Boot Record).