For a company that's been doing it for 6mos+, check out
www.twitter.com - essentially just a tagboard with optional SMS notification. There's no info on their infrastructure, but the user options and how SMS is integrated is at least something to learn from.
Other alternative is to see about using a phone as a distribution point - there are a few .NET programs out there that'll interface with a phone and use that as an SMS sending/receiving tool