I purchased the iPhone 4S and was excited to work with Siri and other voice applications like Vocal (that allows you to voice-operate your Mac with your iPhone). I was well aware of the limitations of Siri for Europe at this point in time, but as I'm working internationally I'm writing, texting and emailing in English a lot.
But I didn't find the microphone button on my keyboard.
After a few experiments I found the answer. Like all of these things it's simple when you know it. If you're not interested in the process, jump to SOLUTION.
Since I'm writing in two languages I installed both the Dutch and an English keyboard. When you do this a button showing a little globe appears on your keyboard (system wide) that allows you to switch dictionaries by pushing the globe button.
First I thought that that was the problem, so I deleted the English keyboard. The globe button disappeared but no microphone.
I reinstalled the English keyboard and deleted the Dutch keyboard: the microphone button appeared!
I added the Dutch keyboard again: the globe button appeared and the mic button was right next to it.
Detail: when I first did this I had the 'Hardware Keyboard Layout' set to Dutch and the mic button wasn't there. It only appeared when I switched to U.S. layout.
But when I played with it a little more this didn't seem to make any difference. Maybe I triggered something that made it work.
Maybe the whole thing occurs because Siri/dictation isn't available for the Dutch language yet.
So the solution is: first install the English keyboard (Settings>General> Keyboard>International Keyboards). And only after you've done that install the Dutch keyboard as the second keyboard. Now both the globe and the mic are there.
If this doesn't work, go to Settings>General> Keyboard>International Keyboards again, and set the Hardare Keyboard Layout of the Dutch keyboard to U.S.
Hopefully this helps if anyone else is confronted with the same problem