Though I get the feeling I'm going to regret this, let's go to the sources:
"On a failed save, the target takes 10d6+40 force damage. If this damage reduces the target to 0 hit points, it is disintegrated." (PH, p.233)
"When you transform, you assume the beast's hit points and Hit Dice. When you revert to your normal form, you return to the number of hit points you had before you transformed. However, if you revert as a result of dropping to 0 hit points, any excess damage carries over to your normal form." (PH, p.67)
In this case, it's pretty clear that the 'target' of the spell is the druid in beast form, not the beast form itself. For that reason, the spell would only disintegrate the druid if it reduced her to 0 hit points after reverting to her normal form.
Polymorph uses the same language in its description (PH, p.266); so again, the target of the Disintegrate spell would not be the polymorphed form, but the creature in polymorphed form -- the target would only be disintegrated if it is reduced to 0 hit points after being returned to its normal form.
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Pauper