This case does more to scold the police and prosecutors in Missouri for laziness than it does to actually set any precedent. You'll see what I mean.
So McNeely was stopped for speeding and weaving. The next part of this looks like any other DUI investigation: bloodshot eyes, slurred speech, smell of alcohol, suspect admits to having two beers, has difficulty standing, and does poorly on FSTs. And McNeely gets arrested. Since this is his third DUI, this one was a felony under Missouri law.
On the way to the station, McNeely says that he'll refuse a breath test. So the officer goes to the hospital, where McNeely refuses a blood test. For some reason, the officer has blood drawn from McNeely anyway. The officer did not try to obtain a warrant, and there was really nothing special about this DUI. He would later testify that he didn't try to obtain a warrant because he didn't think it was legally necessary (seems like he hasn't read Gant. The courts don't like it when we act like that).
Unsurprisingly, the trial court suppressed the evidence. And this is where it gets dumb. The prosecution appealed this case all the way to the US Supreme Court. The prosecution's argument was that the natural dissipation of alcohol in the blood stream automatically constitutes exigent circumstances. Under this proposed standard, no other factual background would be necessary, no other aggravating circumstances, just the fact that drunk people don't stay drunk forever would mean that every DUI arrestee could be subjected to a warrantless, non-consensual blood draw.
The Supreme Court describes exigent circumstances as cases where "there is a compelling need for official action and no time to secure a warrant." Most Fourth Amendment questions (including exigency) are decided by looking at the totality of the circumstances, which means that the outcome of each case depends on a lot of case-specific information. If you change the facts then you change the outcome, and questions like these don't often lend themselves to bright-line rules.
But none of those details were properly before the court in this case; the prosecution's sole argument was that EVERY drunk driving investigation involves exigent circumstances based solely on the dissipation of alcohol in the bloodstream. The Supreme Court disagreed, and held that it's not that simple. In some cases, dissipation of alcohol would still create exigency, but you need more than just that one fact. The suppression of the evidence in this case was affirmed.