Which unix flavor or linux distro...

[Hyperblaze]

Would you be more inclined to have on a usb key.

And please give reasons why.

I've got FreeBSD 5.4 on a usb key I have for rescue operations.

 

[SleepWalkerX]

Depends on the size of my usb thumb drive. I would probably get Slax. I like how its modular so I can grab the necessary drivers and packages I need

 

[imported_LoKe]

You could get anything on a usb key. Ubuntu 6.06, FTW! =]

 

[n0cmonkey]

I haven't run into a situation where I wish I had one.