List of iPhone voice commands? (prior to Siri / iPhone 4S)
via http://atmac.org/iphone-voice-commands

Phone Commands

Call a contact
“Call” or “dial” plus the person’s name or nickname as entered into your address book. Optionally add the phone number type at the end, such as “home”, “work”, “mobile”. For example, “Dial John Smith Work” or “Call Mum”.

Call a number
“Call” or “Dial” plus the number, just as you would recite it usually. For example, “Dial 555 9592″.

Make a correction
Say “not that one”, “wrong”, “nope”, “no”, or “not that”.

Music Commands

Start music
Say “play” or “play music”.

Play from a specific playlist
Say “play playlist” and the name of the playlist. For example, “Play playlist gym songs”.

Play from a specific album
Say “play album” plus the name of the album. For example “Play album Dark Side Of The Moon”.

Play from a specific artist
Say “play artist” plus the name of the artist. For example, “Play artist Pink Floyd”. The alternate “Play songs by” plus the name of the artist has also been reported to work.

Pause music
Say “pause” or “pause music”.

Skip to next song
Say “next song”. The shorter “next” has also been reported to work.

Go back to previous song
Say “previous song”.

Shuffle the current playlist
Just say “shuffle”.

Turn on Genius feature
Say “Genius”, “play more like this”, or “play more songs like this”.

Get information about the current track
Say “what’s playing”, or for more specific information say “what song is this”, “who sings this song”, or “who is this song by”.

Misc Commands

Ask the time 
Say “what is the time” or “what time is it”.

Cancel voice control
Simply say “cancel”. With iOS 4  you can also say “stop”.

Get help
Say “help”.