Update on my experience of installing Windows 7 & BOOTCAMP on my 3TB Fusion Drive iMac 27".
Bottom line is that I did it (I think) but it was not as straightforward as the instructions provided by Apple.

Everything worked fine right up to point of actually installing Windows using my external Superdrive - which basically caused the installation of Windows to come to a screeching halt - i think because when the iMac switched over to the windows install procedure, the drivers for my superdrive where no longer available, and the diskdrive just became a cold slab of metal, not doing anything with my disk trapped inside. I got a black screen.
So I rebooted to OS X and checked out my BOOTCAMP, which now had a Windows partition, but no windows!.
So this time I used my WINDOWS ISO image to create a bootable install of windows on a flash drive using the facility within BOOTCAMP. BOOTCAMP again began the process, but it wouldn't let me simply install windows on the exisiting WINDOWS/BOOTCAMP partition. It wanted me to first delete the BOOTCAMP partition and 'uninstall' windows 7 (which hadn't actually been installed anway!!)
So I did that using BOOTCAMP, and looked at my drive. All seemed fine, and I had once again gained my full 3TB (just over actually with the SSD Fusion element).
So I tried again. This time I got to the windows install part, and I got a black screen again - but this time with a blinking cursor???? Did a quick check online (using my laptop) and discovered that this was because I had 'too many things plugged into my USB ports' !!!! aggghhhhhh!!!
At this point very frustrated - but I unplugged everything from my USB ports (Time Machine Drive, Superdrive) and all I had plugged in now was a USB mouse and the windows install application on a USB Flash Drive.
When I rebooted and started BOOTCAMP I discovered that I once again had to 'uninstall windows 7' (Which hadn't installed anyway) and remove the BOOTCAMP Partition, and then start the BOOTCAMP install again.
So did this, and this time all went well. Windows actually began to install.
I then panicked because I didn't have a USB keyboard plugged in, and remembered that at some point in the process I am going to need to input details, serial number key, etc. But when i got to that point the wireless apple keyboard worked for some reason???
Anyway, windows installed, and then I ran the Apple BOOTCAMP windows drivers package I had downloaded previously to another USB stick.
And I now appear to have WIndows 7 Bootcamped on my 3TB Fusion Drive and seems to bootup and work as expected.
The only caveat is that in order to acheive this functionality on a 3TB drive, APPLE seem to have had to develop an approach which lumps the BOOTCAMP partition in the middle of your hard drive. If you delete the BOOTCAMP Partition, you then get what appears like two separate paritions before and after the space the BOOTCAMP was on, but it is seen by OS X as one volume.
My Windows install though seems really big at 55GB - not quite sure why its that big???

