One thing people who say "Well I should be far enough away that I should be good" seem to keep overlooking: The environmental and economic effects.
Even if you sit comfy on the other side of the world, even if the blast is limited to where only the surrounding American areas are affected, a big enough explosion would send the entire world into a state of emergency. Most agriculture in the USA would be completely wiped out. Even a huge chunk of central and south american agriculture would likely feel the effects of a large enough eruption. These are global exports we're talking. Sudden/expected loss of supply and inevitable panic would cause stores to go empty and/or costs for food to skyrocket.
The US economy would be completely crippled beyond anything we'd ever seen. Even in places where the most they get is a few inches of ash, many businesses will likely have to close shop. The international transportation industry (commercial delivery trucks, railroad, airlines, etc) would be brought to a screeching halt due to many transportation methods being blocked, destroyed, or unsafe.
The USA housing bubble was a domino effect that threw the entire global economy into a recession. Imagine what would happen if the entire US economy collapsed in a matter of hours.
The fact is that, even if scientists realize that Yellowstone is really on its way to blowing, we'll never know about it until either right at the moment or just before, because the global panic would cause just as much (if not more) harm than the actual eruption.