15. A computer-readable storage device storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving audio data encoding an utterance, and data specifying a time when the utterance was spoken; determining, for each of multiple communications that were initiated by a user of a mobile device, a time when the communication was initiated or received; determining, for each of the multiple communications, a similarity score based on a similarity between the time when the communication was initiated or received, and the time when the utterance was spoken; determining, for each of multiple contacts associated with the user, a probability associated with the contact based at least on the similarity score for the communications that were initiated or received; weighting a contact disambiguation grammar according to the probabilities; and processing the audio data using the contact disambiguation grammar to select a particular contact.
15. A computer-readable storage device storing software comprising instructions executable by one or more computers which, upon such execution, cause the one or more computers to perform operations comprising: receiving audio data encoding an utterance, and data specifying a time when the utterance was spoken; determining, for each of multiple communications that were initiated by a user of a mobile device, a time when the communication was initiated or received; determining, for each of the multiple communications, a similarity score based on a similarity between the time when the communication was initiated or received, and the time when the utterance was spoken; determining, for each of multiple contacts associated with the user, a probability associated with the contact based at least on the similarity score for the communications that were initiated or received; weighting a contact disambiguation grammar according to the probabilities; and processing the audio data using the contact disambiguation grammar to select a particular contact. 16. The device of claim 15 , wherein the operations comprise: receiving data specifying a date when the utterance was spoken; determining, for each of the multiple communications that were initiated by the user of the mobile device, a date when the communication was initiated or received; and wherein the respective similarity score is determined further based on a similarity between the date when the communication was initiated or received, and the date when the utterance was spoken.