Help sending SMS messages in bulk

I'm interested in developing an application that will allow me to send a short text message to a list of mobile phone numbers.
What's the best way of doing this?
Basically, what I want to do is provide a service where a client will send me his/her list of phone numbers and the text message. I will then take care of sending those messages through my program. It could be a few, but it can also be thousands.
I know you can do it through email, but I've also heard of different terms for this (GSRM, SMPP server, etc...).

Anyone have additional information regarding this technology?

Thanks.

As far as I know you need an SMS gateway, this can be provided by telco's or mobile phone networks. I'm unsure about the US but in NZ they charge per message.