Could iMessage be ported to ios4?

Siri wasn't ported over to iOS 4. iMessage could, but you'd have to really know what you're doing and you'd have to make a iOS 4 client for it. iMessage runs on the same backend as FaceTime.

 

You should upgrade to iOS5 anyway.

There are already apps that require it, and more are on the way.

 

True. By the way, I think farmermac was thinking about the "porting" of Siri to the iPhone 4 which was in the news earlier in the week. However, the porting was terrible because it was very laggy. There was huge discussions on Macrumors and everywhere it was on the Internet.

However, I don't think iMessage is worth porting. You might as well upgrade to iOS 5 and be happy with it. I don't understand what is keeping you away from iOS 5. Unless your iPod Touch cannot support iOS 5, I understand....