Activitate Voice Dictation from Bluetooth Keyboard
As of iOS 5.1, Voice Dictation is not capable of being started from external keyboards. 

There is a workaround.  The solution is that when you want to dictate, press the ⏏ key (eject) on your external keyboard.  This should bring up the virtual keyboard on the screen, from which you can then start dictation.

Note: This key doesn't work like other function keys until you have tapped into a text entry field and have the flashing cursor.