.08% was selected as the magic number because scientific tests determined that at this level a person's coordination is noticeably affected. Driving is considered a divided attention task, which means that a driver has many things to focus on at the same time - the road, other cars, their speed, street signs, pedestrians, bicyclists, the radio, etc. Alcohol delays a person's reaction time and if you are reading a street sign and look back to see the person in front of you is stopping, alcohol in your system can delay the signal from your brain to your leg to slam on the brakes.
In the late 1990s, the states were split in how they applied DWI laws. Some states had a .10% level, some a .08% level, and two states had no laws concerning the minimum level for intoxication. There was a strong national push to make uniform standards at that time and the federal government then dangled a carrot for the states. States that lowered their BAC limit to .08% would be eligible for a pool of money. The result, we have a .08% limit nationwide now. (You can check out more facts surrounding this from the GAO report here).
Will the limit get lower than .08%? Across the globe, the limits vary from zero tolerance to .10% (thank you Swaziland). As long as there are DWI related deaths there will always be pressure to lower the BAC limit. Depending on the person and conditions such as an empty stomach, even one drink can impair a person. Groups looking to prevent drunk driving will point to the lower fatalities that have resulted since the legal limit was lowered, and enforcement and penalties increased.
I've already heard from bar and restaurant owning friends who said they will close up shop if the BAC drops to .05%. At that level, one drink could cause a person to be over the limit. It appears there will be a fight if the level is to drop. But if the federal government offers another financial incentive for every state to lower the limit, then the fight might just be a minor skirmish in this cash-strapped economy.