This is straying a little off from what you asked (and I'm afraid it got long!), but while we're definitely not unschoolers here by anyone's definition, I think we do manage to follow/develop DS's interests and passions. I describe our homeschool style as "collaborative" - it's not child led, it's not grownup led... we're kind of like a little committee. DS has things he wants out of it, I have things I want out of it, we negotiate and come to an agreement, tweak things, renegotiate, see how they work.... It all kind of comes together.
Sometimes it's obvious that we're responding to a specific interest -- DS has asked to do Marine Biology next year. So we will. On the other hand, if we're doing Marine Biology next year I insist that we get through a certain amount of regular biology first so we're not reinventing the wheel on the way. So that's me. And I've asked that he contribute a certain amount to the planning -- locations and species to focus on -- and that he get that to me by March at the latest so I can figure out our schedule (some things are seasonal - I don't want to hear in September that he wishes we could have volunteered at the turtle sanctuary during breeding season!). And then once it's on the calendar, it's an assignment. No more negotiating unless it's really dire.
Similar with his annual science fair project. I'll suggest that he choose something in keeping with our year's theme/topic (Biology this year), he'll manage to turn that into something he's interested in (bio-TECH), and then he knows I'll drive him all over creation if necessary and buy all kinds of ridiculous materials, but he has to get the work done and write it up. period.
Then there are things that I think he might like but he hasn't discovered for himself yet. Next year I'm considering Economics. He's absolutely certain he wants to be an engineer when he grows up. He might be right... but he's also ten, and I see some other possibilities in his range of interests and talents. So while the idea of Economics almost certainly isn't going to occur to him for a long time, I'm betting that once he knows what it's all about he'll be intrigued. I could be wrong. If he makes it through a semester without getting hooked I'll probably let it go, but as long as it's on the calendar it's required. It won't kill him anyway.
I think unschooling can be an excellent thing. I think it would even work for us to a certain extent (he wouldn't sit in front of the TV for very long without finding something better to do), except that for us I like our way better. We both like a predictable structure, and we both like to keep schoolwork as its own little thing. We start our day, we work, we finish, we put away our work, we go have fun. It's very much school at home, but it's a school that offers exactly the right classes at exactly the right time with exactly the right accomodations.
