1. A speech recognition system configured for use with a portable electronic device including telephonic capabilities, the portable electronic device comprising a user interface configured to receive speech from a speaker as input, the speech recognition system comprising: at least one processor programmed to: determine that the received speech includes an item in an acoustic vocabulary of the speech recognition system, wherein the item is not associated with a valid command construct of an open embedded grammar of the speech recognition system, wherein the open embedded grammar includes a plurality of valid command constructs, each of which includes at least one command word for performing an action on the portable electronic device in combination with one or more words in the acoustic vocabulary of the speech recognition system; determine a first confidence score for the item, wherein the first confidence score is determined based, at least in part, on an acoustic score for the item, wherein the acoustic score for the item is determined based, at least in part, on at least a portion of the received speech, wherein the first confidence score is determined based, at least in part, on a language model score for the item, wherein the language model score for the item is determined based, at least in part, on at least a portion of the received speech, and wherein the first confidence score is determined based, at least in part, on a sum of the acoustic score for the item and the language model score for the item; determine a second confidence score for at least one word in a valid command construct of the open embedded grammar, wherein the second confidence score is determined based, at least in part, on an acoustic score for the at least one word in the valid command construct, wherein the acoustic score for the at least one word in the valid command construct is determined based, at least in part, on the at least a portion of the received speech, wherein the second confidence score is determined based, at least in part, on a language model score for the at least one word in the valid command construct, wherein the language model score for the at least one word in the valid command construct is determined based, at least in part, on the at least a portion of the received speech, and wherein and the second confidence score is determined based, at least in part, on a sum of the acoustic score for the at least one word in the valid command construct and the language model score for the at least one word in the valid command construct; and add a new command construct to the open embedded grammar based, at least in part, on a comparison of the first confidence score and the second confidence score, wherein the new command construct includes the item.