In that case, I'd use the tutorials at Pendrivelinux. They include persistent installs, so you can keep things like bookmarks.
The tutorial for persistent Ubuntu install from live cd is at http://www.pendrivelinux.com/2008/05...a-the-live-cd/
I found this only worked properly if you installed LiLO as per the fix at the bottom.
Once this is done it should boot OK on pretty much any machine that has a BIOS capable of booting from USB regardless of OS - it completely bypasses the existing OS so it doesn't matter what's on there, it's just a matter of having hardware that Ubuntu supports. I haven't tried it on a Mac yet.