I had the same questions about Ubuntu. You can work with the SD card just by plugging phone in - don't need a driver. You can even root the Nexus S using Ubuntu.
That said -since I'm not too good with the command line, I found it easier to root with XP. I've done 2 phones. Most stuff is written for Windows as it's the most common OS. Mini Kies used to upgrade doesn't have a Linux version as far as I know.
Actually, any computer used by you alone is a PC. Personal Computer. That definition applies to all OS.
General usage changed that to Mac vs Windows disregarding Linux, BeOS, OS2, etc.