Why? The equivalent of FaceTime for text messages exists today, and it's any IM app with push notifications. Meebo and Whatsapp come to mind.

IM requires separate logins to be exchanged, they exist on partitioned networks, etc. IM's handy and has its place, but it's not going to replace SMS.

You should try Whatsup. Based on contact list phone numbers - no login exchange required. Very SMS like in nature. The gf and I use it as a SMS replacement and it works well.

Same here. However, there's way more lag in these apps (both msgs arriving when not in the app as well as actually just starting the app) than the built in SMS app from Apple.