I don't think anyone has received a 2012 iMac with the Fusion option, perhaps later this week.
However, as to your question:
I have on a DIY Fusion. Twice ... but in different ways.
On a late 2011 MacBook Pro I created a standard "Fusion" drive with a 256GB SSD and a 750GB hard disk (oem). I then created a BootCamp partition and installed Windows. As Apple documents state, you can create a single BootCamp partition on a Fusion drive, and it will be created on the hard disk. Windows then boots and runs just like it did when the MBPro had only a hard disk ... at hard disk speeds (and the original disk is a 7200rpm drive. Fusion is only active when running under OS X, so it will offer no speed benefit to a BootCamp Windows experience. Fusion will enhance a virtual machine (Parallels, VMware, etc.) windows since it is running under OS X.
On a 2011 Mac Mini I had already upgraded with a 240GB SSD and 500GB hard disk. I had installed Windows using BootCamp on a partition of the SSD for speed. When I created the "Fusion" drive, I converted and joined only the OS X partition of the SSD with the entire hard disk, and reloaded the OS X system from backup. Both operating systems work fine, and fast, and I can switch between them in the normal fashions.
Unfortunately, at this time the iMac Fusion options only allow for a 128GB SSD which is too small to subdivide into a Windows Partition. Hopefully 3rd party upgrades will become available soon, or Apple might offer more optional configurations for storage in the future.
-howard