Google’s Matt Cutts just posted a video on YouTube answering the question, “how many search algorithm changes were made in 2009?” In response to that question, Matt said Google likely makes a change per day to the search algorithm. They don’t necessarily release those changes each day, but they will release them in batches. But overall, he hopes to average at least one change per day to the algorithm. He said in 2009, they probably had between 350 to 400 or so changed to the search algorithm.
I’m sure many of these are minor tweaks but it shows that the Google search algorithm is ever changing.