I read an article about Text over IP on the The Inquirer recently. While this sounds like it could be the answer to SMS charges, for me it forgets some of the fundamental features of SMS that made it so successful, interruptability and push support.
SMS utilises the signalling channel on mobile networks a channel that is designed to support interruption. It's primary use is to control call setup and tear down so any messages sent through it must get through as soon as possible. SMS messaging has leveraged this feature to provide a very rapid delivery of messages to handsets wherever they are. Even to the point where you can be interrupted by an SMS message while you are on a call.
Push is also supported out of the box. For the signalling messages to get through to the handset, the network has to know where it is at any given time. IP in mobile networks on the otherhand is a layer on top of a layer with no constant, unique link between the IP address used by the device to access the Internet and the device itself. Multiple mobile devices will share a small number IP addresses for this purpose.
To circumvent this limitation, services like Hotxt require an application on the device that polls for messages on a periodic basis. The poll interval used is really a balancing act between the perception of immediacy and the inefficency of polling when, for the most part, no messages will be waiting.
The BlackBerry service on the otherhand has a centralised server and proprietary technology that allows them to push emails to devices only when they arrive. Great if you have a BlackBerry, but if you're like the overwhelming majority of mobile phone users not.
The AQL representative suggested that all Nokia had to do was to release the APIs and a new era in messaging would dawn. This does forget another powerful party in this market, the network operator. As demonstrated by Orange UK and Vodafone UK recently disabling VoIP on the Nokia N95, they're completely prepared to override a handset's capabilities to restrict how their networks are used.
If you can cope with the limitations of Text over IP then solutions are out there already. In my opinion, it's just not the complete package.