If my Warforged Monk has the Jaws of Death warforged feat (Races of Eberron, p. 119), which provides a secondary natural weapon, can I use it after Decisive Strike?
Natural attacks typically can't be used with a decisive strike
The alternative class feature decisive strike begins by saying, "As a full-round action, make one attack with an unarmed strike or a special monk weapon…" (Player's Handbook II 51 and emphasis mine), and this makes using it a new and unique full-round action. This means that the decisive strike's phrasing disallows its use in conjunction with typical natural attacks like a bite or gore. A typical creature can no more also make natural attacks when making a decisive strike than it can make natural attacks when casting a spell with casting time of 1 full round.
Compare the alternative class feature decisive strike to the typical monk ability that decisive strike replaces, the extraordinary ability flurry of blows (Player's Handbook 40) that, to employ it, only mandates that the monk take the action full attack (143). Anything the monk can do during a full attack—like also making attacks with typical natural weapons—can be done when making a flurry of blows.
However, this isn't to say that a DM should in his own campaigns continue to disallow natural attacks in conjunction with the alternative class feature decisive strike. In many campaigns, monks need all the help they can get, and a house rule that changes decisive strike to a full attack might be just the thing that sees the monk pull his weight alongside his fighter and samurai compatriots.