I remember hearing about bonded diamonds a while ago. They are pitched heavily by Diamond Cutters International, which I think I can safely assert does NOT enjoy a good reputation here on the forums. "Crook," "scammer" and "convicted felon" are expressions that have been bandied about within the same threads that discuss Mr. Cuellar and his company. Do a search for some of the keywords for more info ("bonded," "cutters" or "fred" will yield lots of info here in the forums).
Anyway, a "bonded" diamond (as described by Mr. Cuellar) is ostensibly a diamond with a bulletproof warranty attached to it. A bonded diamond is supposed to be (among other things) warranted against breakage (free replacement), have a lifetime exchange/trade-in guarantee, be guaranteed to be untreated "natural" diamond, and to even have a "market crash" guarantee that would reimburse you if the diamond ever loses value due to a diamond market crash.
To me, that last one is a huge red flag that screams SCAM to me. What kind of a legitimate business offers such risky assurances? It's a swell idea, but my guess would be that if the market ever crashed, the bonded diamond dealer would be the first one in line at the courthouse to file their bankruptcy protection paperwork, having had the documents at the ready since the day they started offering bonded stones.
Anyway, since bonded diamonds seem to be more expensive than non-bonded diamonds I would avoid them. The benefits are sketchy at best, and in reality many of the supposed perks are already offered by many dealers (trade-in policies, untreated stones, etc.) without the shadiness that seems to accompany the bonded stones.
My $0.02...
-Tim