Adding the extract at the end of the boil is pretty new practice, but it's been embraced by most now. I was reading a Zymurgy article about a year ago with Charlie Papazian where Charlie said he had never heard of this, though, so maybe it's not quite as common as I thought!
It makes sense if you think about it. Not that the extract has already been boiled (it has) but when you do a partial boil of wort, you get a superthick wort that actually darkens and exhibits maillard reactions. You don't get that in a full boil or in the all-grain version. So holding back some of the extract until the end actually more closely simulates the consistency, gravity, and color of the all-grain or full boil wort. That thinking is what me start doing this about 5-6 years ago.
After I tried it by adding some of the extract at 15 minutes left in the boil, the wort was lighter in color, with a less "cooked extract" taste to it. I thought that was encouraging, so I ended up using more extract at the very end of the boil (at flame out) and less in the beginning. I never tried not using any extract in the beginning, but I often used DME at the beginning and LME at the end if the recipe used both. My extract beers turned out really good that way.
I haven't made an extract beer since late 2006 or so, but I took careful notes of the results and do remember the difference in the quality. My late extract addition beers were more like a typical commercial beer without as much darkening and with a better less extract-y flavor.
I think some homebrew stores may not be as open to change as some others. I believe it's Northern Brewer who has their instructions in some of the kits for adding about 1/2 of the extract near the end of the boil.
It's worth a try, just to see if someone likes the results better or not. There is no downside at all, except that getting DME to dissolve in hot wort at the end can be a PITA.
Keep in mind that there is really one reason to boil an extract beer for an hour- to maximize hops utilization. Since the extract has been processed already, there would be nothing "magical" that would happen to the extract in boiling it. In AG brewing, the boil is necessary to kill bacteria, reduce volume, get a hot break, evaporate SMM (DMS precursors), and so on. None of that is necessary with extract.