Some speakers will image quite well up against a wall. That is, IF they are designed for such a situation and you have NOTHING in between them. This means no racks, tv's, etc... that protrude beyond or close to the front plane of the speaker.
In order to achieve this, the speakers would normally have to have a very focused radiation pattern. As Duke mentions, baffle shape and dimensions will also come into play and the use of some type of "acoustic blanket" or sound damping material ( felt, foam, etc.. ) on the baffle also helps. In effect, the "acoustic blanket" helps minimize stray radiation and reflections, making the wall behind them less "attached" to the baffle acoustically.
Other than imaging and soundstage, Duke brings up another very valid point. That is, he talked about woofer loading / room reinforcement when a speaker is placed near a room boundary. Placing a woofer closer to the floor, near a corner, up against a back wall, etc... will add bass reinforcement. Obviously, this could skew the tonal balance of a speaker if it were not designed for such placement. If the speakers are quite lean sounding though, one can take advantage of the situation to help fill out the bottom end. If the speakers already had reasonable bass response, you could easily end up with too much of a good thing. To top it off, the bass would probably not only be more apparent, but also of poorer quality in terms of definition.
If a speaker already has some form of woofer loading to it, this would make it slightly less susceptible to reinforcement from the back wall. That is part of the reason why the Snell's that Duke mentions could work well in such a situation. Such designs are still susceptible to such a situation, but not as severely as a "standard" design.
Some speakers were even designed for such a situation. The older AR 9's and 90's took room placement into great account when being designed and were recommended for placement up against the wall. These speakers were VERY ground-breaking in many areas and that is why you see many of their design innovations popping up in more current speakers. There are reasons why these and other speakers seem to have "odd" driver placement i.e. having the woofers mounted near the floor and on the sides of the cabinet, etc... Unlike many speakers, these were designed to work with the room taken into consideration and not just tested anechoically. This is discussed to a very great extent in the owners manual for these speakers, which just so happens to be 50 pages thick !!! AR provided various graphs and a lot of technical information for different speaker locations in terms of room boundary reinforcement. They also showed how these speakers would work in comparison to a "standard" speaker put into the same situation. While one could take this as a marketing ploy, one's ears and experience could verify much of the information that they presented in these graphs.
The other speakers that instantly came to mind for such a situation were the old Allison's. Roy Allison believed in using the room and not fighting it. As such, his designs are a little unconventional but can be made to work where other designs fall short.
Since you mentioned Kef's, there was an older design that might work well for you that they made. I want to say it was the 105, but can't remember exactly. This had an LED that one could see if you were sitting directly on axis with the tweeters. They were of a very focused field design, which should give you what you want for your specific situation. Whether or not you like the individual characteristics of this speaker or others like it remains to be seen. Sean
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