The beauty of iMessage is that there is virtually no setup. It autodetects if the other person is on an iOS 5 device & sends your message via iMessage instead of a text. The end user does nothing different, iOS does all the work.
Other platforms would have to rewrite their default texting app & if there was ever an update to iMessage, all the other platforms would have to update them. Basically, it would be a nightmare.
It's a nice competitive advantage Apple has over WP7, webOS & Android.
Lolwut? I guess you've never heard of Google Talk. The messages go everywhere that you're signed in with your Google account and all messages automatically logged for later retrieval (you can turn this feature off if you want).
Oh yeah, you can access it from your Android tablet, phone, or any web browser.
This means that I can start a chat with someone as i'm finishing up on the computer, continue the same conversation on my phone leaving my house, and finish the conversation on my computer at work. Seamlessly.
I'm sure that iMessage will be useful, but calling it a 'competitive advantage' is a bit of a stretch.