I hate the idea of using speach recognition for security. One of my friends was locked off his computer cause he his voice did "not" match the voice recorded for about an hour.
What if you get a cold?
There are security programs that can do this, just make sure that they are very good and have a backdoor "Just in case." If its not a good program it will be very tiresome. The more you have to train the program the better.
As of right now, speech recognition is a joke. A gimmick to make XP look more valueable. It stills has too much tweaking that the average user will have a hard time getting it going. They've been working on text to speech software for some time now. By the time you wrote an email with it you'll have so many corrections to make that it will take longer than just typing it in the first place. Just forget about until they can get it right.