Major Players on the top side of the logic board include:

Intel Core i5 Processor-2557M with integrated Intel HD 3000 graphics

Intel E78296 01PB10 / E116A746 SLJ4K Platform Controller Hub. We're guessing this includes an integrated Thunderbolt controller. It's not this part, but it's similar.

Genesys Logic 822 SD-slot controller

DSL2310 L123TA46 Eagle Ridge SFF Thunderbolt controller

Linear Technology 3857

Shifting to integrated graphics on the processor freed up a lot of room on the board—enough for Apple to add the sizeable Thunderbolt-capable PCH.

If you're interested in more information on Intel's mobile chipsets, Wikipedia has a lot of interesting information.