Only glitch with your idea is that it would allow people to 'virtually' cut in line using SMS to get ahead of everyone else that waited outside in line if I am understanding this correctly, in that you want something like the 'take a number' system used in service locations where you pull the number from a roll, and then wait for the "now Serving" LED to show your number, except you want people to be able to SMS to get their number sent to them, and then have a SMS notice when they are within 10 numbers from being served.
Good thing in theory if everyone has cell phones with SMS and doesn't have to pay for it, but what about people that don't.
Think this would be a better thing to ask in somewhere like the iPhone apps development area on other sites.
Some of these things like Top 10 can be done with SQL using a database, same as registration by SMS, but the load your idea would place on the system would be inefficient, and again it doesn't take into account everyone without SMS that might have camped outside your business to get a good place in line.
They have tried systems like this for selling event tickets, and now it is easy to abuse the system and "sell out" the event by electronically manipulating the system and cheat everyone else out of tickets so you can auction them off online.
But best of luck, hopefully I have given you some other ideas to try.... or put the 'take a number' roll outside, and an LED Display in the window with "NOW SERVING" on it so people would know.