The economics of this decision are pretty clear: used, but pristine case tops are available on eBay for about $70, whereas AppleCare warranty extensions are $350. You also have to consider the inconvenience of being without your laptop for days (weeks?) while it's being repaired, together with the possibility that your hard drive will be erased by a technician running the computer through a battery of tests.

Having decided to replace the part myself, I spent a good deal of time reading through the excellent disassembly guide at ifixit. The following pictures are from my own disassembly:

The basic strategy is to remove the over 1 dozen screws which bind the top of the lower-half case to the bathtub-shaped tray containing the motherboard, optical drive and other items. Some of these screws are accessible via the battery compartment, as shown above, whereas others are external on the right and left sides:

Once all the screws are removed, you can gently lift up the top of the case, which interestingly enough contains the entire keyboard assembly as an integrated part:

The thin silver band shown linking the case to the motherboard is the only cable which carries the trackpad and keyboard data to the motherboard. As you can see from the shot above, the MacBook unfortunately accumulates quite a bit of dust by virtue of its interior fans. My guess is that leaving the machine running on the floor for days at a time contributes significantly to this problem. I took the opportunity of having the entire case open to perform a bit of spring cleaning. Below is the motherboard looking a bit nicer:

After reassembling everything the front of the notebook was no longer marred by the chipped edge. We'll see how long this new case top lasts...