Argonians definitely generate body heat as they do move around a lot. Also I don't think a totally cold-blooded creature would have fast enough metabolism to be able to have high brain activity.
It's still going to be interesting to see if Argonians have thicker scales or bodies in Skyrim.
"Argonians are out." said Todd Howard in a interview, "They just didn't fit with what we were doing, so we had to cut them out." After the interview, Todd went back to the office to find out that it was burned to the ground......
The Shadowscales would hit Todd's office in an instant if he would say that.
Edit:
Guys, the evidence is right within Morrowind. At many instances, Argonians in that game refer to humans as warm-blooded and soft-skinned, clearly implying that Argonians are neither. They sure as hell are cold blooded. This is set by lore.
Their skin is likely harder, and blood flow slower, but they still generate body heat.