I've been using Mandriva's new Linux-distribution-on-a-USB-stick, Mandriva Flash, and most impressive it is too. On a small 2GB USB key, you get a fully portable Linux desktop comprising the Linux Kernel 2.6.17 plus KDE 3.5.4, Mozilla Firefox 1.5.06, OpenOffice 2.0.3, KMPlayer, Adobe Flash Player and a number of other applications and plug-ins.
I'll post a full review soon, but the main advantages over a similar Live CD distro are that there's a spare 1GB of capacity on the USB stick for downloading additional software and storing your own data, and that any CD drive on a host system will be available for use rather than occupied by the Live CD.
Not every PC has a BIOS that allows booting from a USB drive, but there is a workaround involving a small CD image on the stick that you burn onto a disc and boot from with the USB stick inserted. Stay tuned for the full report.