[susan53]

The problem is that it's a Latin word, and in Latin it's plural : sing datum, pl. data

Data has been incorporated into English, but of course is not recognisable as a plural (unless you know Latin). So people tend to use it in the same way as "information" (ie as an uncountable), because of the similarity of meaning:

The information/data is unreliable.

However, because it is originally a countable plural, other people (especially those who know Latin) use it that way :

The data are unreliable.

This is slightly less common, but only slightly. A Google battle turned up the following results :

data is : 2,110,000,000
data are : 1,900,000,000

So in short, you can use it either way. But if the person you're talking to is a Professor of Latin, you'll impress more by choosing the plural