My understanding about APS vs. GUID where booting Macs is concerned is this:
You need to have a GUID-based destination when running the OS X installer on Intel machines. You need to have an APS-based destination when running the OS X installer on PPC machines. However, once OS X has been installed, you can copy the system from a GUID partition to an APS partition and it can be used to boot the machine. We have a number of Firewire drives with APS partitioning that contain both Intel and PPC booting systems, either copies of various DVDs, or hand-built loads that we use for a variety of purposes.
I wasn't describing things quite right: it's not a matter of the drive being fixed or removable, it's just that I tend to have systems with bootable internal system+user drives and removable external data drives. To keep things simple, the internal drive wants to be GUID-based on Intel machines (so that you can install systems on to it) and the external drive wants to be APS-based on all machines (so that you can plug it in to any Mac). Time Machine, however, may be an exception to the rule; it looks like it requires GUID partitioning on Intel Macs.
The issue of APS and GUID where booting Leopard is concerned is discussed here:
http://www.tidbits.com/webx?14@@.3c7e511e/8