After some research, it looks like this company has such a thing: http://www.proliphix.com/

The Carrier Infinity system can also potentially do this: http://web.consumerelectronicsnet.com/articles/viewarticle.jsp?id=86816 (but it doesn't look like you're preparing to replace the entire system).

Hopefully someone else knows of one and has experience with it. There don't seem to be cheap solutions.

A Venstar 1900 with either an Insteon comms module or a telephone interface could to this. The phone interface is not internet but the Insteon is when combined with an ISY-26 network controller.
I have the Insteon system, 2 of the Venstar thermostats and the work very well. Also do humidity control with fan speed control along with dual fuel capability and full remote monitoring and setpoint adjustment.

Internet access Stats and monitoring

http://www.ewccontrols.com/acrobat/izt.pdf

Here is a link that might be useful: EWC

I think this is what your looking for

http://www.smarthome.com/30532.html

They have other similar products.

No affiliation. No experience with these systems either.