"Data is" or "data are"?

Answer: "Data is." "Data" is a singular noun.

Yes, I know that "data," in Latin, is the plural form of "datum." But we're speaking English, not that ancient dead language, and "data" has taken on a new identity in the modern world.

In Latin, "datum" means "something given." In English, it means "a piece of information," or "a datapoint." A few decades ago, the plural "data" was equivalent to today's "datapoints." But now the word has the more nebulous meaning of "information." Perhaps the computer age gives us such a huge number of datapoints to contend with that we no longer think of data as a congregation of individual points. Instead, they fade into a huge, amorphous cloud. "Data" is now a mass noun that is used as a singular. And no one says "datum" anymore.

Plural forms of former Latin words often cause confusion today. Is it "stadiums" or "stadia"? "Aquariums" or "aquaria"? 

The answer: just add an "s" to these words as you would any other. This might not have been correct years ago, but it's definitely the right way now. When we English speakers snag a word from another language, we savor its foreigness for a while and keep its native plural. But when it moves permanently into our corral and becomes an everyday part of speech, it deserves no special privileges. Aquarium and stadium definitely fall into that category by now, as do many others.

Now, we don't have to throw out all Latin plurals. Many terms in academia, for example, reflect the era in which Latin was the language of the learned. I'm sure we'd all prefer to be a member of an alumni organization instead of an alumnuses organization. It's fun to use those lofty -i and -a plurals in that setting, and it gives the words some color. But in ordinary speech, stick with good old Engllish-style plurals.