Rather than match your voice against its general database, Google's Voice Search (a.k.a. Voice Actions), after downloading a just-released update, can now gradually learn from your particular speech.
Google says they'll learn over time, with subtle accuracy improvements that "begin fairly quickly and... build over time". The data they collect is tied into your Google account, too — great for those transferring between phones, and you can wipe it out any time through your Google Dashboard. The update is free in the Market, requires Android 2.2 or later.