OpenEars is an excellent option: http://www.politepix.com/openears/
You can create a custom list of words to listen for in your app and when recognized there are some convenient callbacks that provide the word and accuracy level at which it was identified.
You can also take a look at Nuance: http://www.nuancemobiledeveloper.com/public/index.php?task=home
From my experience if you are looking to identify key words open ears is a better solution, if you need to transcribe a paragraph nuance has the advantage.