We can only speculate, but in all likelihood based on what little we know so far:
Ivy Bridge i7 quad-core CPU
SSD primary storage possibly expandable with additional hdd but possibly not
8GB DDR-1600 RAM expandable to 32gb/64gb
Some sort of anti-reflective front panel
NVidia GTX-680 GPU
TB, USB3, and no FW800 ports
802.11ac or n and gigabit networking
I'm with Seamaster however in that I currently doubt there will be an Ivy Bridge iMac and instead I expect Apple to skip straight to Haswell next summer. I expect this because it is not what I want, and therefore it is what will surely happen. (shrug)