Apple uses Tier 1 direct manufactured memory which is supposed to have superior reliability compared to 2nd Tier memory resold under different names.
Just search for "RAM Kernel Panic" on Google and you can see why they're so picky.
Not to start a whole discussion on memory quality, rather trying to head this one off ...
Apple does use top-quality memory, from mfrs like Micron, Samsung, Nanya, and Hynix.
And ... 3rd party memory does vary in quality, but one can get top-quality memory - just as good as what Apple uses - by going to a known-reliable brand or supplier, for a fraction of the cost of Apple-supplied RAM. Crucial has a very good reputation as such, using mostly, perhaps entirely, Micron memory. OWC has a good reputation. There are also other good brands and suppliers. That's why other posters in this thread are so vocal about Apple's prices being so high. Look at reviews for the memory/supplier before you buy (newegg is a great source for reviews of stuff like this). The differences between requirements for Mac vs. PC memory are much smaller (if any?) since the switch to Intel processors, since the motherboards are pretty much the same when it comes to memory access.
The gold standard to find out whether the memory you purchased is top quality is whether memtest (or GUI version
Rember) will run without producing any errors at all. 1st tier should pass, 2nd tier probably won't.
E.g. I used to like memory supplied by Data Memory systems - good price and quality. But, about 5 yrs ago, I bought memory from them for a PowerMac G5 that failed memtest. A replacement batch failed also. Returned it for a refund, bought Crucial, and it passed memtest no problems.
memtest is harder on memory than anything else you are likely to do (i.e. you might run a system for years w/no problems using memory that produces errors under memtest).
There are lots of threads about all of this elsewhere - Google/Yahoo/Bing are your friends.