Yes and no. Google Desktop was a fully contained application; indexing was just an internal mechanism for its own functionality.

NEPOMUK is closer to an actual filesystem, in that it's not user facing; it's a service for other applications, like email clients, media players, etc. This allows them to almost effortlessly¹ share information like user contacts or music metadata.

¹ Apparently, a basic but functional email client can be built on top of the platform in 10 minutes.