I was actually just on a jury a few weeks ago for a DUI case.
There were three things we were asked to determine as part of the charges:
1) Was the defendant "under the influence" of alcohol, and we were instructed that BAC has nothing to do with whether or not alcohol has influenced you. A person with a high tolerance might not be influenced, which a person with no experience drinking and a small stature may be under the influence of alcohol. There was a long description of things that a reasonable person might consider to be under the inlfuence. This was count one of the charges.
2) We were also asked to determine if the prosecution proved that the defendant had a BAC of 0.08 or more (a separate statute, wherein operating a motor vehicle with a BAC of 0.08+ is a crime). This was count 2 of the charges.
3) We were also asked to determine if the prosecution proved the defendant had a BAC of 0.20 or more, in which case this would become an aggravated crime. for the second charge.
So at least in my state, you can get a DUI without blowing over the limit if the prosecution can convince a jury your driving was impaired, caused by the influence of alcohol. The 0.08 limit is just an automatic threshold in which you have committed a crime.