Mi sugerencia

depending on what sort of plea...

I think you might want to provide a bit more context. A plea can go either way (i.e. guilty/not guilty/no contest). I believe this refers to the resolution of a criminal procedure as a result of a plea of not guilty, but cannot be sure. In my experience, it is in Juvenile courts/Children's Court where the term disposition is used. And most often than not is is in direct relation to an agreement between the parties to which the Judge then makes a finding. It is not always a sentnece (sentencia o condena). ALthough I think that in this case it is related to a criminal process. And I would use the term EXPEDITA because it is exactly what expedite means because it refers to the process itself not the ammount of time itr takes. In other words, it is not related to how fats the bureaucracy moves, more rather to the waiver fo some parts of the procedure as a whole to make the process go faster without compromising its integrity.

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Note added at 13 hrs (2007-12-12 04:59:22 GMT)
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Correction: I meant to say that "this refers to the resolution of a criminal procedure as a result of a plea GUILTY". Otherwise if the Plea was Not Guilty the matter would not be expedited and it would be turned over for trial. Disposition can be used in any criminal procedure.
Just keep in mind that in countries where we don't follow Common Law, there might not exist the concept of a Plea Bargain/Agreement and therefore the concept of Disposición might not be applicable. It is always the judge who decides the outcome of the case.

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Note added at 13 hrs (2007-12-12 05:08:52 GMT)
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One more correction: I would lik eto eliminate my answer. What is expedited in this case is the PLEA not the disposition. The disposition is a direct result of the defendant having changed his plea from NOT GUILTY to that of GUILTY. Therefore the correct version in Spanish should be:
Disposición o sentencia como resultado de una declaratoria expedita de culpabilidad.
How's that? I think it is better.