Do you have your heatattached to the same ductwork or do you have something else like baseboard heat? If your heat is using the ductwork then they do sell humidifiers which hook right into the ducts. I just got an estimate and it came in at $450 from my local heating and air guy. I don't know if this system would work with just A/C ducts.

Good luck,
Devildog

Thanks for the reply.

I have forced hot water heating. The duct I am hoping to use was installed during the installation of the A/C.

Here is a link on some excellent humidifers available. Good luck.
http://www.aprilaire.com/category.asp?id=F16F78145781484A8A7C756B87F43AB3

Unless you have some type of heat in the attic duct system, you will not be able to install a humidifier in the duct work. The humidifier requires a source of heated air to absorb the moisture that it is putting into the duct system. Aprilaire does make a self contained humidifier especially for hydronically heated homes. Models 350 and 360.