It's quite experimental and definitely not documented, but we have been using PocketSphinx to do speech recognition with ROS. See the cwru_voice package for source.
If you run the voice.launch file (after changing some of the hardcoded model paths appropriately in whichever node it launches), you should be able to get certain keywords out on the "chatter" topic. As an example, voice.launch should recognize a command to "Open the Door" or "Go to the hallway" and output a keyword on the chatter topic. If you do try it out and have problems, let me know as you would be the first outside our lab to try it that I know of.
Stanford also has a speech package in their repository. EDIT: Thanks to @fergs for finding the Stanford package.
UPDATE: Make sure to take a look at Scott's answer below for a nice tutorial and demo code for getting speech recognition up and running for your own uses.