Sorry, 5.1 on my iPad 2 doesn't show the dictation microphone. Must indigenous to the New iPad!
There Apple goes again, endowing only its newest hardware with a feature that at least some of its older hardware can handle just fine. If it were any other company, I'd hold a grudge--BUT I approve of and enjoy watching Apple take over the world and doing whatever they have to in order to achieve this.
If I had the iPad 2, knowing that the iPad* can take dictation would mean absolutely nothing to me. Its other advances (although there aren't many) far outweigh that IMHO little feature. And Siri on my 4S? I hardly use it. So far the most gratification I've gotten from Siri has been the result of watching some of its parodies on YouTube.
*I'm still in mourning over Apple brutally amputating the "3" designator we'd been expecting. Even "HD" would have been nice. Just consider the sentence: "Samantha Jones has enjoyed her iPad 2 for months, and is now eagerly anticipating upgrading to the iPad."