Don't the external drives have 2nd connections for daisy-chains? All of mine do, even a portable one for laptop drives. It's all plug-n-play, so you should be able to plug in anywhere, if you have a port available. A hub should be just fine if you need that route.
Most do, but I actually have a couple of FW drives that lack pass-through.
Another consideration is bandwidth. As you add more devices to the chain, things do get slower; keep in mind that every device on the chain would be sharing the same lone FW controller. In critical applications where speed is important, expansion to add an additional FW controller (whether it be PCIe or ExpressCard) is crucial. This is one of the areas where the iMac obviously suffers.
But all in all, I'm pretty impressed with these new top-end iMac offerings. Apple could have been using mobile C2Q chips last year and they finally deliver quad-core with the new Lynnfield chips.