That's called the Baader-Meinhof Pehnomenon or the Frequency Illusion. It used to have it's own wikipedia article but now it's just a single sentence in List of Cognitive Biases [0]. Check out this post here though [1].

[0]: http://en.wikipedia.org/wiki/Baader-Meinhof_phenomenon#Frequ... [1]: http://www.damninteresting.com/the-baader-meinhof-phenomenon...