As others have mentioned, placement up against a wall is detrimental to imaging - specifically, to depth of image. Even with controlled radiation pattern speakers like JoshK suggests, the imaging is still significantly better when the speakers are pulled out into the room a little ways.
I must disagree with Response Audio about one minor point. A front-facing port does not necessarily make a speaker more wall-placement friendly. Let me explain (I doubt any of this will be news to Response Audio - I'm just clarifying my position for others):
The port's output will get maximal boundary reinforcement if it's located very close to the wall behind the speakers, and if the port is located less than one diameter from the wall then the tuning can actually be changed by the wall. The ideal in my opinion would be a port that tunes the box lower than it would normally be tuned for placement out in the room, and that is accomplished by either increasing the length or decreasing the diameter of the port. This is one of the nice things about a reflex box (at least from a designer's standpoint) - you can choose a very wide range of low frequency response shapes, so you can optimize for the speaker's acoustic environment if you know in advance what it will be. So by tuning the box maybe a half-octave lower than normal and placing it near the wall, what would have been bass boom is transformed into bass extension. And a rear-firing port takes maximal advantage of this.
Now what if the speaker is placed so close to the wall that the tuning of the rear-firing port is affected? Not a problem - you see, the wall would lengthen the effective port length and lower the tuning frequency - which is the direction you want to go.
Some manufacturers (including yours truly) use a port system that allows the user to adjust the port length, thus tuning the speaker for its environment. As mentioned, a long port will tune the speaker lower and is thus better suited for against the wall placement. But unfortunately a long port is also more likely to introduce midrange coloration from residual backwave midrange energy off the back of the woofer's cone (think of talking through a cardboard tube). By putting the port on the rear of the box, any midrange coloration that emerges from the port is less likely to be audible than if the port were on the front of the box.
Finally, what about the case where the port length is fixed? Well then it might still be possible to change the tuning by inserting a longer pvc or cardboard "sleeve" into the port, and using a few wraps of electrical tape to get a good friction-fit. This might not look real good cosmetically, and the midrange coloration issue exists, so once again we're better off with a rear-facing port. So don't write off a fixed-port speaker - with a little ingenuity, you can still change the port length. And, I'll let you in on a dirty little secret - the port length the designer chose may not be the optimum length for your room, with your speaker and listening position locations, and your amplifier and listening style.
Now lest you assume that yours truly is the only one crazy enough to advocate a rear-firing port for a speaker placed up against a wall or even in a corner, I stole the idea from Audio Note. Of course, maybe we're both crazy.
Duke
