Sending text messages is not simple. Not for software implementors, in any case. In the traditional telco environment, there is the sprawling SS7 MAP specification that defines a whole variety of information that can be sent between networks. Some functions are dedicated to SMS message exchange.

The bad news is that the Internet doesn't make life any easier. The SIMPLE protocol (which deals with presence and IM for SIP devices) now has more than 1000 pages of protocol definition. The other IETF effort, XMPP, is just as bad with its own boat load of extensions built on XML.

So will all IM/text systems one day be able to interoperate? Will we ever be able to SMS to GoogleTalk and OCS, while having a text conversation to friends on Gizmo, AIM, Yahoo and Skype? Will Facebook seamlessly exchange messages with Twitter, Jaiku, Fring and Raketu? The answer is surely "yes". After all, email has come from nothing to near 100% coverage in only 15 years, and now everything uses the same protocol, although arguably not the most rigorous one. It will be interesting to see what system ends up on the top of the pile for IM, and also to see if it takes another 15 years (which I doubt) or perhaps just 5.