Colloquially, data is. But to the data scientists who live and die by p-values and correlation coefficients, data are. That is, "data" is technically the plural form of the singular noun "datum,", but "data" has come to be synonymous with a collection of data points—i.e., data shows, data says, data tells us. And when was the last time you heard anyone say "datum?"
A recent Wall Street Journal style guide blog post made public their their official position on the matter:
As usage has evolved from the word’s origin as the Latin plural of datum, singular verbs now are often used to refer to collections of information: Little data is available to support the conclusions.
Otherwise, generally continue to use the plural: Data are still being collected.
Simon Rogers picked up the WSJ announcement on The Guardian's Datablog with a quick look at the term's history and some heated tweets on the subject. Rogers concludes:
For what it's worth, I can confidently say that this will probably be the only time I ever write the word "datum" in a Datablog post. Data as a plural term may be the proper usage but language evolves and we want to write in terms that everyone understands—and that don't seem ridiculous.So, when you're addressing the general public, it's probably safe to assume that data is. But if you're in a room full of statisticians, be wise to the likelihood that data are.