I flash rather frequently, just last night in fact, and have had 3 or 4 failures, and as others pointed out, it will effectively kill your motherboard until you reprogram the chip, either hot flashing it or in an EPROM programmer which not too many people have. If you do do it, there are a couple of ways. If you are using a command prompt driven flash program, make sure you boot to a totally clean DOS prompt. Easiest way is to create a bootable floppy from windows and boot with it. Make sure no memory managers are present on the floppy. Just the command prompt loaded. Have the flash program and code on the floppy as well and you can run them from there. Make sure you have the correct BIOS code as incorrect code will load and has just as bad an affect as screwing up the BIOS chip. Most flash programs have a self check now that flags you if the code is not for the board. However, the code might be right and the flasher might say otherwise, but if this happens make darn sure you know what you are doing if you decide to flash. When flashing, make sure there is nothing that might shut down the system possible, i.e. thunderstorms causing electrical outages. If you follow all these rules you should be able to flash successfully. Last night I flashed a new beta BIOS for my Asus CUSL2-C and have found definite improvements in the system's stability and OCing capability.
Finally, some of the motherboard manufacturers now have windows based flash programs. Asus does and that is what I am using now. Simplifies the process a bit. However, as one other poster pointed out, don't do the live update where the program goes out to the net to download the code. This is still new and unreliable. Get the flash code before hand and download it to your system and use that file for the job. Good luck.