Don't fret - you probably didn't even know that you already HAVE a voice control in your current iPhone. It's simply called "Voice Control", and here's what it can do.
Use Voice Control: Press and hold the Home button until the Voice Control screen appears and you hear a beep. You can also press and hold the center button on the iPhone earphones.
Call someone in contacts Say “call” or “dial,” then say the name of the person. If the person has more than one phone number, you can add “home” or “mobile,” for example.
Make a FaceTime call to someone in contacts (iPhone 4) Say “FaceTime,” then say the name of the person. If the person has more than one phone number, you can add “home” or “mobile,” for example.
Dial a number Say “call” or “dial,” then say the number.
Control music playback Say “play” or “play music.” To pause, say “pause” or “pause music.” You can also say “next song” or “previous song.”
Play an album, artist, or playlist Say “play,” then say “album,” “artist,” or “playlist” and the name.
Shuffle the current playlist Say “shuffle.”
Find out more about the currently playing song Say “what’s playing,” “what song is this,” “who sings this song,” or “who is this song by.”
Use Genius to play similar songs Say “Genius,” “play more like this,” or “play more songs like this.”
Find out the current time Say “what time is it?” or “what is the time?”
Cancel Voice Control Say “cancel” or “stop.” You may enjoy reading a blog that I like - called Paul's iPad, he has great content and a topical article on Siri on the iPad2 here.