no you can't. the miniDP port on the new iMac only receives video in from other Macs like the macbook. miniDP tech is very different from HDMI, so a miniDP-to-HDMI converter wont bring in video from the PS3 to the iMac display. not even from xbox360 or Wii.

from my understanding it's a hardware limitation so dont bother waiting for an update to fix this. probably you should get a smaller monitor to play PS3... maybe 24".