Anandtech has a very good article on the Mac Fusion Drive, and how it works and who it is good for.
Sounds great to me personally, and I would love one for my MacPro if that is possible (probably not).
I like the idea of the pinning and moving, as I have about 1 TB currently on my main system hard drive, so the 3TB with Fusion would be awesome. Hopefully a 3rd party can figure this out and have 256MB instead of 128MB with the 3TB.
I know, I know I should just go SSD, but I just don’t have enough hard drive space. I have all 4 drives in my MacPro with 2TB hard drives (would love to have 3’s some day) and no room to add an SSD at all (even have 2 optical drives, a DVD-R and a Blu-Ray burner as well)..