To clarify: that's a plugin that makes possible to receive and send SMS with your phone number on Messages.app (Mac version) like you could automatically if your phone was an iPhone, nothing else.
Will iMessage ever be available on multiple platforms? As a longtime iPhone user, currently with an Android phone, I really wish iMessages would be available everywhere.
If history shows us anything, iMessage is such as asset to Apple's toolbox, like Blackberry Messenger was to RIM, that Apple would likely rather go down (unlikely), then open up the iMessage platform.
IIRC that "open standard" part was tacked on at the 11th hour by people who were unaware of what that actually entailed. (Not to excuse Apple for saying this)
