[oudplayer]

I also seem to remember there being issues with low-frequency response (which can be boosted by up to 6db), which some monitors compensate for with a switchable low-cut or low-shelving filter that's meant to be used when soffit mounted. In speaker designs lacking such a filter, you'd probably need to run the signal through a filter prior to the amplification, otherwise it's going to be boomy. Of the monitors we have in our studio (NHT A20/S20, Meyer HD1, Dynaudio BM15a, Genelec 1031a [but not for long...]), only the Genelecs are immediately usable in a soffited situation - the others recommend against it specifically.

Oh, and there's the problem with horning if your speakers are too close to the back wall in a soffit configuration. Thus, the sheer amount of real-estate you lose in a well-constructed soffit configuration may dissuade you from this approach...