SOl_Zehn is right on! With ceiling speakers, there is no way to get around the "inherent positioning problems" that he describes if you want to any serious 2 channel listening. The websites he has provided are also excellent places to start.
For ceiling speakers, I've done a whole house installation with a total of 24 mid-fi Polk Audio in-ceiling speakers specifically for the low level background listening that Sol_Zehn describes, but find that I rarely use it. At first, it was 16 speakers, and then I went back and added 8 more here and there to tame the hot spots. In large open areas the speaker pairs are set to mono rather than stereo to eliminate the left-right directionality, but in a small bedroom the stereo effect is pretty cool since it's right above the bed. I'm using the Niles Audio IntelliControl system, in-wall keypads, and a pair of Niles 12-channel amps. The felxibility of this system is nice and it impresses visitors, but overall, it was a very disappointing experience in terms of sonics. For this system, I ended up using an old Onkyo receiver and a Sony 200 disc CD changer and relatively cheap Kimber Kwik 12 speaker wire.
For in-walls. it all depends on your application, your budget, and how much you are willing to sacrifice in sound reproduction relative to the convenience gained in terms of having a low-profile, high WAF (wife acceptance factor) design to your HT. I've used 2 pairs of B&W Signature 7 in-wall speakers (retail for about $1,200 per pair but can be had for less on eBay every now and then) and do not find them lacking in any significant way other than the inherent limitation of their fixed positioning. I've been surprised at how well they handle complex musical passages in certain movies and would not hesitate to recommend them to anyone looking for serious in-walls. There are several other high-end in-walls (Niles has several well designed models, as does Triad and some other companies as well). When placed properly, there is very little sacrificed in HT sonics relative to comparably priced monitors. I have not heard anything about the Dynaudio in-walls.