The lighting is the biggest difference. In the Playstation 3 version the shadows cast across the opening sequence are sharper, there's less subtlety in the way the light is diffused. During the gunfight that takes place outside in the parking lot there seems to be a lot less smoke and dust on the PS3 version. When my teammates fire, their barrels don't spew out smoke that quickly dissipates. In the PC version the entire parking lot seems to be clouded with debris, smoke and dust, in the PS3 version the same scene seems clearer, cleaner.
But these are the sort of differences you'd only notice if you were looking for them. At first blush, the Playstation 3 version of Battlefield 3 is every bit as impressive as the computer version. The fidelity of the urban battlefield is overwhelming, the nuance of details found in the buildings, cars and make up of the city serve as a sort of camouflage for the action that unravels during the gunfight. It's hard to spot enemies until they open fire.