Apple has released many firmware updates: http://support.apple.com/kb/HT1237?viewlocale=en_USgood point...it's impossible to get a BIOS update for a Mac lmao
It's EFI firmware since Apple never used PC-style "BIOS" on its Intel or any other models.
But the general question of whether you can upgrade your Mac Pro firmware later, it's been done with pretty easy hacks: http://forum.netkas.org/index.php/topic,852.0.html As you can see from there, many people have been able to upgrade CPUs on the prior models.