Yes, this is one of the new aluminum Macbook's. I had considered the OS version as a possible problem when trying to boot from the USB drive, so did update the OS on the external drive to 10.5.5 and it did not make a difference.
It won't make a difference, if you are updating to 10.5.5 on an older machine. You won't get the drivers you need. Boot from your 10.5.5 HD on the old machine, and take a look at the build number. (You get that by selecting "About this Mac" from the Apple menu, then clicking on the line that says "OS X 10.5.5" in the window that comes up.) Compare that build number with the build number on the new Mac. They should be different.
If you use Carbon Copy Cloner (or SuperDuper) to make an exact copy of your new Macbook's hard drive on the USB drive, then it should be bootable.
I am also aware that these new MB's use a completely new set of drivers, but question why I can boot into Tiger from my firewire drive on a machine that is running Leopard internally (my Mac Mini), not that I'm trying to boot into Tiger on this new machine.
Your mini supports Tiger. Tiger has all the software needed to run the Mini, so you can boot a drive with Tiger on it. It doesn't matter that your internal drive has Leopard installed.
Just want to be able to boot from ANY external drive.
For now, you need to make a copy of your internal disk. When 10.5.6 comes out, I expect it will contain all of the latest drivers. At that point you should be able to do an update, even from an old system, and have a bootable disk.