I am simply wondering, out of pure curiosity, if any of rM's members have ever had a BIOS Rootkit.
Is it true that a hacker would have to have physical access to a computer to install one?
Not necessarily. Theoretically it can be done remotely if the attacker gains root/admin access. Some manufacturers (or most) offer tools that you can use to upgrade your BIOS from within Windows. The attacker has to do the same thing.
What does that mean (inject code)?
Alter the original code.
With that being said, a BIOS rootkit is pretty sophisticated, and as long as malware authors have much easier options to infect your system (like kernel-mode drivers or MBR) I doubt you'll ever see one in the wild. The BIOS from one manufacturer differs from the others so a universal BIOS rootkit doesn't really exist.