That's Montessori in a nutshell. "Follow the child." Although, to be fair, the Montessori philosophy is to factor in the kids' interest, pace of learning, etc. into the lesson plan, not to operate without one.
To an extent I play it by ear. My middle one is into Macromedia Flash animation, so I let her indulge her interest. On the other hand, I also make her do algebra whether she's in the mood or not. Essentially, if they have an interest in something that's reasonably academicly-related, I let them run with it, while also making sure that the basics (reading, writing, math) are addressed