A HDMI to display port adapter should work with the iMac in question since it supports Target Display Mode.
Here is an example of an HDMI to Display Port adapter the StarTech HDMI to DisplayPort Active Converter. Lastly video wise you would need a display port to mini display port cable to plug into the iMac.
As for the audio you can use a optical audio cable from the PS3 optical audio out to the line in port on the back of the iMac.
Or you can use a Digital Audio To Analog Audio Converter such as this and a send the output to the iMac, amplifier, or computer speakers with the proper adapter cable.
Word of caution: Depending on the speed of the active HDMI to DisplayPort converter, the audio and video may not be in sync.
Note: that is will not work with any iMac that has Thunderbolt since these macs can only provide DisplayPort output capabilities, and that may be spotty at best according to Thunderbolt port to mini Display-Port cable, where there were reports of graininess.