From what i could tell (and how flimsy it felt when i was using it), its plastic.
I am an Apple technician and this is incorrect. Any standard SATA drive (SATA, SATAII or SATA3) will work in any iMac. It is recommended you use a SATAII as this is native to the iMac, SATA3 will derate to SATAII and SATA will run only at the drives supported SATA speed.
Granted the firmware is different on the Apple drive that comes in the machine or in our hard drive upgrades, but the standard firmware with your aftermarket drive will still work as good!