OSX from a thumb drive is pretty cool. Unfortunately, you have to boot normally, and then use the Disk Utility to switch the boot drive, then reboot... I've also seen a method where a bootable CD is used to mount the flash drive and then continue the system startup from that...
Booting OS X from a USB Hard drive does work for me. I use SuperDuper, which makes a bootable image of your main hard drive on a USB drive. If you ever loose your main hard drive, you just boot from the USB Hard drive, and you're up and running with all your usual applications and files. It made it very easy for me to upgrade my MacBook's hard drive from 80GB to 200GB. I put the new 200GB hard drive in a USB enclosure, used SuperDuper to make it a bootable copy of my main drive, then swapped the drives. The other day I got Puppy Linux running off a usb thumb drive. It was pretty cool, but the old hardware that would really work well with Puppy doesn't have a bios that can boot from a stick. Darn. Puppy also doesn't have any Python, which is a deal-breaker for me. The Puppy on a Stick wouldn't boot on my Intel MacBook, (holding down opt during boot didn't show the usb device.) Tomorrow I'll try the official 8.04 Ubuntu... I saw an article that showed how you can boot a machine from the Ubuntu disk, then do an install to a USB flash stick... and then boot from it. Aaah... here's the article. http://www.pendrivelinux.com/2008/04/14/ubuntu-804-usb-hard-drive-install/ Ubuntu On A Stick is just too cool... I love Knoppix on a DVD for system recovery, but CDs and DVDs get scratched over time. I'm looking forward to the day that machines have a very fast 8GB of flash right on the system bus... so the system could boot and run very quickly, and only use hard drives for the big, infrequently accessed stuff. Cheers, -Jim
