I am trying to develop a Windows based application that can receive
SMS messages from a cell phone (North America based for now) and then
send an SMS response back to the originating cell phone according to
certain business rules.

I think I need a service provider that:
(1)  will provide one or more SMS numbers at which my customers can
send me a message
(2)  will provide APIs that will allow me to write an application to
receive and parse an incoming SMS message and send a response message
(3)  is inexpensive and does not charge on a per message basis

Can you provide me a list of service providers that meet those requirements?

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Clarification of Question by jzalesne-ga on 08 Jun 2004 16:35 PDT

To clarify:

I need to be able to receive and send sms messages from any sms
enabled US cell phone regardless of carrier (assuming, of course, that
the carrier supports SMS on its network).

You can get this service from http://www.truesenses.com

Your customers can send a SMS to their access number with a keyword in
front. The keyword is the one you have configured to be forwarded to
your application.

The incoming service is inexpensive and charged at a flatrate.
Response messages are however charged on a volume basis.

SMS Everywhere does not charge on per-message basis.
http://www.smseverywhere.com/
They have an API and you can send test messages from
http://www.smseverywhere.com/send.htm

The article http://www.developershome.com/sms/smsGatewayProvComp.asp and
http://www.developershome.com/sms/howToChooseSMSGateway.asp may help you a bit.

> (3)  is inexpensive and does not charge on a per message basis

Almost all SMS service providers charge fees on a per message basis.
If you do not want them to charge on a per message basis, you can
contact the SMS service provider's staff and discuss the details with
them. They should be able to provide you a custom solution.