If you do insist on trying to use a flash drive to boot, it can be done with Knoppix on computers that properly support it. Puppy Linux and DSL are also good candidates, and one of these has utilities that configure the flash drive very cleanly
I already have DSL booting on a usb drive. I am using it right now. I agree with both of you concerning usb boot difficulties. My external usb drive is actually a laptop drive in a usb case. At bootup its registeration is slow and the boot process just passes it by and doesn't consider it to be a bootable option even though the bios supports it, because of the registeration lag. The work-around with DSL is quite simple. Just make a boot floppy and boot from it. When the prompt comes up just type in "dsl fromusb". The same goes if using the DSL CD. Both will wait extra time until the usb drive is active and then boot from it.

I "assumed" I'd be able to accomplish the same with Knoppix. I could not get a boot in either the Beginner or Knoppix hard drive installs. In the beginner, if I tried to boot the usb via the cdrom it simply failed to find the drive. In the Knoppix install it failed and indicated it could not find /KNOPPIX/KNOPPIX. To boot I used the lines "knoppix fromhd=/dev/sda1" "knoppix bootfrom=/dev/sda1" "knoppix fromhd" and "knoppix fromusb" Couldn't get anything to boot the usb drive and haven't been able to determine any line to get it to boot.

Uniquely, when knoppix fails to boot off the usb drive and continues on with a cd boot, if I reboot the box while it's in the process of that the usb drive boots on restart. Then I run into a huge problem. I've run into it 3 times now. If I try to do anything as root it fails because the root password is rejected. Trust me, I did not give the incorrect password on 3 different hard drive installs.