I have often said that I am never surprised by anything that Google does. They take some chances at times and some of the moves they make don't work. But they do make enough of the right moves.
There is all types of speculation as to why Google is interested in Feedburner. Many wouldn't be surprised to see Google integrate Feedburner into Blogger, Google Reader, Google Analytics and others.
I have seen a wide range on how many feeds Feedburner has. I've seen it from 400,000 up to 750,000. It is a large number whatever the actual number is and Feedburner is said to have over 60 million feed subscriptions.
The first thing that comes to mind with most people is that Google is interested in getting ads into RSS feeds.
Google is full of surprises, so it is difficult to know everything they would have in mind for wanting Feedburner.
Here is another article with a some more information
MediaPost Publications - Google's Feedburner Grab Would Impact Personalized Search, Analytics - 05/25/2007