The move continues Google's pursuit of social search.
If you're not familiar, Aardvark is a Q&A search engine of sorts. Users ask questions, which will be sent to members who have expressed interest in answering questions according to categories (sports, movies, etc.). You get a direct response from those who answer the question. You don't have to go hunting for the answer. Instead, you wait for the answer to come to you.
Aardvark is a type of social search. We've seen many social searches that are basically similar to a Wiki. Results are perfected by the "crowd." But Aardvark facilitates Q&A within the crowd.
Google is pursuing social search, but has not gone the Wiki-esque route. So far, their social search consists of including relevant results that your networks on Facebook, Twitter, etc (info you must proactively provide Google) have shared or expressed interest in. Then this week came Google Buzz, which most people still seem to be figuring out how it will work and if it will take off.
The Aardvark acquisition is another piece in the social puzzle that Google appears to still be figuring out for themselves.