Most of Google's services have an API that allow developers to create their own applications around them. Will Google Base be any different? Why wouldn't application developers or webmasters fuse their applications with a robust, fault tolerant database with an infrastructure that would cost hundreds of thousands to build in-house... especially if it's free? Not only would developers jump at the ease and cost effectiveness of using such an API, but they could also gain exposure by allowing people to find their content by simply searching.
The domain fusion.google.com is one of the more recent subdomains that has popped up, and if I was betting on its use, my money would be on a "Base" integration API. There are current subdomains that behave the same as fusion.google.com such as calendar.google.com which is rumored to be Google's "calendar" service that will help organize people's lives. A past example of these types of domains is talk.google.com... just before "Google Talk" was released, this domain would also redirect to www.google.com.
When it comes to subdomains, we cannot guarantee what they will be used for, but in most cases Google has something in store for them. Because of the unpredictable nature of Google, "Fusion" might be something down the road on a track far off of what I have just described. Even so, I think Google would agree that "Base" is an excellent candidate for a free API and expect that effort will be put into this in the future if not right away.