Usually. A couple of exceptions:
PPC Macs cannot boot from GPT partition tables/hard drives created on Intel Macs.
The brand-new Macs released after 10.5.6 require the media that came with them. That means the nVidia chipset Mini, Nehalem Mac Pros, and nVidia chipset iMacs. Once 10.5.7 is released, they should be able to boot off of a 10.5.7 hard drive no sweat.
To make a truly universal boot HDD, install on a PPC Mac. Intel Macs can read Apple Partition Table disks, and the Leopard retail DVD is truly universal - installs on PPC and Intel.
Each time you move the HDD, the OS recognizes taht it's been moved to a new computer (based on MAC address/serial number, IIRC) and it recreates the Extensions.mkext - the boot kernel extension cache - so it might take 30-45 more seconds to boot up. Regardless, this is better than multiple reboots as hardware is recognized in Windows when you try to do something similar.
I do not understand what I do. For what I want to do I do not do, but what I hate I do.