11. An adaptive speech recognition system comprising: one or more computer processors; a language model component comprising a language model that includes a context-free grammar (CFG) and a plurality of back-off grammar (BOG) rules, the BOG rules each comprising a rule for producing a recognition result from a user utterance and being derived from one or more grammar rules of the CFG that specify words and patterns of words, wherein the language model component obtains recognition results, using at least one of the computer processors, for user utterances by processing the user utterances with the CFG during a first pass and then processing a portion of the use utterances that comprise out of grammar utterances with the BOG rules during a second pass; and a grammar refining component that iteratively modifies the language model, using at least one of the computer processors, based on a data indicator that identifies the most frequently occurring out of grammar utterances handled by the BOG rules, wherein modifying the language model comprises incorporating the most frequently occurring out of grammar utterances into the CFG.
15. The system of claim 11 , wherein iteratively modifying the language model comprises automatically adjusting the CFG used on a determination of minimal edit distance.