In iOS 6, Siri is smart enough to open your apps for you. Just speak the command "Open Mail," and Mail will open. This works for built-in and downloaded apps.
[kirkmc adds: It's about time, frankly. I've been extremely disappointed in Siri; it hardly ever works for me. I hope this works, because this will save time when I want to open an app that's not on my home screen, or buried in a folder.]
Mac OS X Hints
http://hints.macworld.com/article.php?story=20120915164716167