They were charging for iMessage over a year ago in what is now called Beeper Cloud. I wonder why Beeper Mini created the ruckus, perhaps because it was iMessage only?
I think it is because this new Beeper Mini spoofs a Mac, whereas the older Beeper Cloud used a real Mac in a datacenter to forward your messages to you. Personally, I don't think that Apple would have any leg to stand on if they went after the Beeper Cloud on-a-real-Mac implementation.