I have an idea for a service to replace SMS (Short Messaging Service) with a "SMS over IP" structure. Basically a mobile Java / J2ME application connects to a central server with persistent socket. Using the established socket, the server can send data to the phone. Therefore, people can send "SMS" (actually only ordinary text data) to the server (using their own connections), and the server can route the message to another account which is already connected to the server.
This is very similar to existing IM concept, but with IM there are too many data to send - people logging in/out, etc., and data cannot be retained / saved if the current user is not connected to the server at the moment (hence "Instant" Messaging).
It has a great potential if SMSes are more expensive compared to GPRS connection (which is the case in Indonesia: Rp 350 per sms vs. Rp 3 per kb). I might want to develop this, but the server cost must be high, as there are very many persistent connections at one moment (which needs an MMORPG class server).