This week on, um "This Week on C9" the guys pointed out Zeep Mobile, a free service for sending and receiving
SMS messages (last 40 characters are reserved for targeted advertising).
They have both Ruby and Python APIs
available on Google code.
Usage from "Getting Started" (Python example):
connection = zeep.sms.connect("<your api key>", "<your secret key>")
connection.send_message("<user_id>", "Surfs up!")
Based on the source code, it looks like someone with even basic web authentication skills could whip up a .NET/Silverlight version...