Obviously it will be easy to circumvent, and would increase cost of new cars as well. The easiest method of circumvention being "drive an older car".
If insurance companies are allowed to provide discounts to drivers with these, there would be an incentive to install them. Eventually they might require them for any cars someone under 18 would drive, since they are traditionally the most expensive to insure and some carriers already refuse to write policies on cars that include a teenage driver.
While there are some obvious issues like "what if I just slammed down the rest of my one beer" or women with hairspray containing alcohol or whatever, I think this would be a good thing as a sanity check to insure you're right that you're OK to drive after you've had a couple.
Of course the way things are going, texting/calling (whether hands free or not) are going to be causing more accidents than drunk drivers before long. Studies have shown that even hands free use is about equal to driving at the US legal limit of .08%. So if they do someday require these, I hope they also block people from using phones in any way while driving too, otherwise you are just fixing the old problem while ignoring the new one.