The problem is that the only two things that deaden sound are mass and distance. There are three acoustic things one might try to do with paint, you can try to block the sound from entering the next room, you can try to absorb high-frequencies or you can try to diffuse high-frequencies. I think you'll find that paint is poorly-suited to all three of these activites.

Consider sound-deadening material like sheetblok from auralex.com It's basically a super-dense 1/8" thick sheet of rubberized foam that weighs 1 pound per square foot and reduces sound transmission by about 27dB on average (more in the high-frequencies, less in the low). In other words, to cover an 8' tall 15' wide wall requires 120 pounds of sheetblok. I personally can't imagine slathering 120 pounds of paint on a wall.

Similarly, acoustic foam for high frequency absorption needs to be very thick to actually absorb significant amounts of sound. 1" thick foam can only make a noticable difference in sounds over 2Khz or so, which is totally tweeter territory in many speakers. To signifactly absorb midrange frequencies, you need foam 3" or 4" thick. The same goes for attempts to diffuse high-frequencies: you need diffusers 3" or 4" thick. That's a lot of paint!

Pretty much the only thing I can imagine paint doing is diffusing very high frequencies when it's laid very unevenly as in the case of the popcorn ceilings Woochifer mentions.

But this much I know for sure: You go buy yerself some home depot paint and repackage it as a "sound absorbing" paint, and some fool audiophile who has no regard for the laws of physics will buy it at a 500% mark-up. That much I GUARANTEE.