I have heard of viruses which install into the BIOS. Could this be a reasonable threat to the secure operation of TAILS?

Definitely. If a BIOS virus modifies e.g. the kernel as it's loaded into memory, there's not much any operating system, Tails included, can do.

What other circumstances could exist which would undermine security before TAILS is even booted

Tails does not defend against:

• Compromised hardware (Tails defends against hardware keyloggers if the user uses the provided on-screen-keyboard)
• Compromised Tails boot media
• Video surveillance
• TEMPEST/Van Eck phreaking

Etc.