I was editing a blog post at work the other day - it may surprise you to learn that writing this blog is not my job - and I amended "the data says" to "the data say" because as everybody pedant knows, "data" is the plural of "datum." I didn't think much more about it.
Then somebody "corrected" me and said it was wrong. I fired back some citation or other and didn't think much more about it.
Then a consultant making a pitch for our business chided us in his pitch presentation on our lousy grammar by citing that very post. (Dear consultant guy: 1. you really should make sure your client is unambiguously in the wrong, and 2. there's a sentence fragment in the middle of your own blog post on the subject of grammar. At least we agree that grammar counts.)
Every geometer knows that two points determine a line, and now - counting the original author - I had at least two smart people saying that data should take a singular verb. I needed to think more about this.
There's plenty of evidence that I was right to write "the data say" but also plenty that "the data says" is not wrong. The estimable Grammar Girl has a good blog post that begins by complicating things - it's not about data being singular or plural; it's about data being or not being a mass noun - and ends with some good guidance about picking your path and sticking to it.
I've come around to a different idea. Using a possibly controversial construction is a no-win situation. You either earn points with antiquarian scholars and sound overwrought, or you please the crowd and sound dull to the nitpicky. It's a bit like the forcefully correct use of "whom." Either way you risk some people thinking you're careless or ignorant. If you're got a sentence, especially a headline or title, that forces the reader to think about whether a word in it is right or wrong, I think it's time to choose a different word or write a different sentence.
Perhaps instead of
...the data say... or ...the data says...
we might try
...the survey says... or ...my research reveals... or ...4 out of 5 dentists agree...
Have I given in to the forces of the incorrect by avoiding the issue? I hope not. I don't suggest anybody stop correcting errors of "you're" vs "your" or allowing "alot" to slide by for "a lot" but when there's room for common usage to differ from correct usage or where usage is evolving, making your readers think about grammar instead of reading and digesting your point is a bad idea.