I am not an expert on this but I will try to take a stab at it......
It is my understanding that in wall speakers are designed to basicaly have the wall that it is in become its' enclosure. The enclosure of a speaker is very important. On a "regular" speaker the enclosure is designed in such a way to work with the the speaker in producing the sound. A "regular" speaker to me should not be placed in a wall simply because it is not designed to do so. If it is a rear ported speaker and it is put in a wall this will also obviously effect the sound. If it is front ported would I do it?.....still no way.....does it mean that it will take a good speaker and make it sound like utter crap if it is in a wall.....probably not but it WILL effect the sound in some way and I would rather have the speaker set up in such a way so the sound I am hearing is what the speaker was designed to sound like. Does this make any sense?? LOL!! Someone please chime in and correct me if I am wrong as I probably am :b .........
In a standard speaker's design, the usually well-defined and harsh edges of the front speaker baffle are important to the overall sound character of the speaker. A speaker flush-mounted in the wall, will not sound the same as when it's positioned correctly, in-room.
I agree with Scott and Evan. That said, I fully expect that a regular speaker mounted in-wall will sound vastly better than a comparably priced “real” in-wall speaker.
I have no clue what your budget is, but I was looking at the Salk Sound website today and saw that they have some speakers that are specifically designed to be used as either in-wall or bookshelf speakers. They have a switch on them that adjusts some settings in the speaker based on how you place them: 1) in-wall, 2) against wall, 3) out from wall.
I've never had the pleasure of hearing any Salk speakers, but most reviews are very, very good.
Another option might be to build in-wall shelves, such as I've seen for TVs and so forth in newer houses, on a smaller scale for the speaker. But if you are going to do a real in-wall installaiont please, use speakers, which were designed for that purpose.