I actually have doubts that there will be any noteworthy level of serious hardware failure (permanent RROD) on the "slim" 360 at a rate higher than that for PS3s and Wiis.
The original Xbox 360 had its infamous RROD issue due to the motherboard design; the connection between the GPU and the motherboard would warp due to the intense heat put out, and some of the contacts would be lost, rendering the console unusable. Each revision has addressed this in some way, either by adding extra cooling, (heatsinks/fans) using die shrinks to cut how hot it ran, or altering the design to more firmly attach the GPU to the board.
Although I'm not 100% positive, what I've seen of the new Xbox 360 motherboard shows a radical redesign, that apparently has the CPU and GPU packaged together, under one giant, monolithic heatsink, with a large fan. This design would likely use a different package type for the chips, one which would make the previous form of failure virtually impossible.
That's not to say that the 360 slim won't have failures; all consoles in the modern era have a failure rate, and even the much-vaunted Wii apparently fails 3-6% of the time. But the old, 15-30% rates seen for the 360 that made it infamous should be a thing of the past now.
As for the hard drive, the Slim's HDD is internal, just like the PS3 and original Xbox's. The plus side is that it might be more possible to replace it with a giant-size one affordably; my bets are that it'll use one of the standard form-factors for 2.5" drives, either the fatter desktop drive, or the thinner laptop formfactor. So modders, say goodbye to your 250GB Xbox 360, and say hello to your new 1 terabyte Xbox 360!