Solaris, Linux and any unix based system is safe, as you would need to be root to access such low level functions and you are never suppose to be logged on root. On mac? Who would target a OS with so few users... and as OSX has some unix bases, I think its protected too due to its architecture. On Windows, if you have 95, 98 or Me you are always logged as an admin so it would be possible to do anything you want. If you have 2000 or xp, you are probably logged in account with admin rights, as we are all so lazy, so no protection, but in Vista, if you let UAC on, you are protected as it will warn you something is going on.
To flash a BIOS, you need a driver or a access to kernel or the possibility to dynamically load a driver to perform interuptions and other commands which must be run on ring 0. Commands vary depending of the bios so there is no universal way to screw a bios, luckily.