An update, I am having problems with damping off.
The cuttings are approx 3", dipped in Clonex rooting hormone. I am trying styrofoam cups, with Miracle Grow potting soil, unheated, 7" humidity dome, in a window with indirect sunlight.
Any thoughts would be appreciated.
I will post this on the propagating forum as well.
Thanks

I would suggest that you put your cuttings on a heating pad and that you make sure they get 14+ hours of light each day.

Thanks Steve,
I'm trying a heating pad and lights as we speak.

I have had much much better success growing cuttings (of any sort) in pearlite, than in soil. If you are still having rotting issues give it a try. If you use clear plastic drinking cups then you can see when the roots are developed enough to re-pot them without pulling them out to check. I have also read in posts from professional growers that Dahlias do not need rooting compound to grow from cuttings.

I would also suggest that you read: Success With Cuttings By Bill McClaren.
http://dahlias.net/seabox/McClaren.htm

There is also a lot of good info on the Colorado Dahlia Society web site.

http://www.dahlias.net/
http://www.dahlias.net/dahwebpg/Propagation/Cuttings.htm

Here is a link that might be useful: Success With Cuttings

For what it's worth I just read a contrary opinion that heating pads provide too much heat for dahlia cuttings and that 70 degrees and no hotter is ideal.