Precisely! The iMacs had an extra internal SATA controller port. You could run some wiring from it to an eSATA port (OWC did this) and get the benefit of an external drive at essentially the same speed an an internal SATA drive. It was faster than FW800 in my experience.
There seems to be some confusion about it not being a standard. SATA is a standard. It is just that there are three generations of it and a SATA 3 drive on a SATA 1 controller is only going to transfer data at SATA 1 rates.
Firewire has often sucked on the Macs even more than usual. Typically they'd use a single bridgeport that couldn't negotiate different speeds, meaning if you populated both FW400 and 800 ports, both would run at 400 speeds. They had a lot of irritating gotchas like that. SATA 1 would transfer at SATA 1 speeds regardless of where you run it. Anyway its bandwidth is higher than that of firewire, and the lack of data conversion takes away some overhead. It should run faster, but the upper bandwidth limit is also much higher. It's not as fast as thunderbolt or mini-SAS, but it's much cheaper than either of those. If thunderbolt came down in cost and a two port setup became standard, it could edge out the need for eSATA. Firewire stopped being ideal a long time ago. It was just there for stability and lack of a better option.
There was a combination port I have sen on a few PCs which could physically accept either a USB plug or an eSATA plug. I have not first hand experience with them however.
That would be cool, although I've never seen an external device that would be compliant with such a thing unless a special cable that delivers via eSATA with a usb connector at one end and eSATA connector at the other actually exists.