The first shipments of Apple's new 27-inch iMacs have begun arriving at the doorsteps of early-bird buyers, and MacRumors forum user R.OG has posted one of the Web's first teardowns of the flagship desktop. The internals follow the same general layout as the 21.5-inch iMac, which we commented on last month, but there are a few notable differences.

The most obvious is that the 27-inch iMac has room for a full 3.5-inch desktop hard disk drive. The one pictured is an OEM version of the Western Digital WD10EALX, a 7200 RPM Blue drive. The 21.5-inch iMac, by contrast, uses a 2.5-inch HGST Travelstar 5K1000 HDD, spinning at 5400 RPM.

The images posted by R.OG reveal that like the 21.5-inch iMac, the 27-inch iMac's GPU shares space with the CPU on the logic board. The CPU and GPU heat sink support braces are clearly visible on the back of the logic board. Previous iMacs used an MXM daughterboard for their video cards, coupled with an extensive heat pipe and heat sink assembly to channel heat away from it.

The Kepler-based Nvidia GPUs on the new generation of iMacs are more miserly with their thermal envelope, and Apple has engineered a way to cool the entire system with a single fan, even when the iMac is configured with the top build-to-order CPU and GPU options (an Ivy Bridge i7-3770 and a GeForce GTX 680MX, respectively).

The speakers are larger than on the 21.5-inch iMac, owing to there being more space inside, and the power supply daughterboard below the HDD is larger than in the 21.5-inch iMac. As with its smaller sibling, the 27-inch iMac has wireless antennae scattered around the edge of the enclosure, and it's secured together with sticky foam. One of the challenges for would-be DIY enthusiasts is getting through the foam to get to the iMac's internals; a guitar pick apparently works well as a foam cutter. However, even more of a challenge than that is how to get the iMac back together after you're done fiddling with its guts.

Interestingly, Mac reseller and parts shop Other World Computing has begun selling a RAM upgrade kit for 21.5-inch iMacs which includes a package of sticky foam for resealing iMacs. The 27-inch iMac doesn't need a RAM upgrade kit—its RAM is user-accessible through a port on the rear—but we speculate a similar "Open & Close Your iMac" parts kit will be available soon.

Compared to the mess of components the previous-generation 27-inch iMac contained, the new 27-inch iMac is downright minimalist inside. This is no doubt a combination of Johnny Ive's industrial design insight, coupled with CEO Tim Cook's legendary (and legendarily ruthless) supply chain management expertise.

We're expecting to receive our 27-inch iMac early next week. Andrew Cunningham has already reviewed the 21.5-inch model, but we'll have a comparative review of the 27-inch version up shortly after UPS drops it off.