Since the Buffyverse seems to correlate the soul with a conscience, a sence of guilt, and the capacity for selfless love, then I would say that Dawn does have a soul or a reasonable facsimile. However, given that Joss is an avowed and very outspoken atheist, I seriously doubt that he would see Heaven as being anywhere in Dawn's future. Maybe one of those "heaven dimensions" that Tara mentioned Buffy being in when she was dead, but I'm not sure that that is quite the same thing.
My thoughts exactly...