We got two types of comfrey this year from Horizon Herbs:
Comfrey, Russian Live Root Cutting (Bocking 14 Cultivar), organic 6 root bag, discounted, organic

7/26/11: Russian comfrey, African sumac (came back from ground after it froze), sugar sorghum and native plant with ultra perfume smell

The Russian Comfrey roots did best close to the garden hose leak where it was also very shaded and protected from the wind.  I think they all 6 roots lived at least for a while after I planted them into our various gardens.  However, the comfrey with lots of water had leaves about 18 or 20″ long, while the others had around 6 inch leaves.  The big one almost flowered.

And we got seeds: Comfrey, True (Symphytum officinalis) seeds, organic
I got some starts from the seeds, but have only seen 2 or 3 plants lately, in our greenhouse.  The leaves are about  4 inches long now.

What's the difference between this plant and true comfrey (Symphytum officinalis)?
The Bocking 14 cultivar of Russian Comfrey (Symphytum x uplandicum) is a sterile hybrid that will not self-seed and is extremely robust and vigorous.  The true comfrey (Symphytum officinalis) is a bit less vigorous of a grower, has more elongated leaves and (I think) prettier flowers, and does indeed make seed.  Although both types of comfrey (Russian and True) are useful for making medicine and making compost, in an ideal world one would use the bocking cultivar for producing large amounts of biomass for permaculture gardens, composting, and animal feed, and one would use the true comfrey (Symphytum officinalis) for medicinal purposes.  Again, both types (and other species as well) are used interchangeably in agriculture and in medicine.

I've read that comfrey is bad for the kidneys and liver and I have used purchased comfrey root powder on my teeth for several weeks.  It definitely helps with small cavities and I've used only very little.
Now I just found a little more clarification about potential dangers of comfrey at Wise Woman Wisdom … Comfrey, Symphytum uplandica x:

… Every time I mention comfrey, someone asks if it isn't “unsafe.” When I identify with comfrey, I feel like a persecuted witch wrongly accused of evil-doing. Comfrey has so much to offer as an aid to health and healing. How did such a wonderful green ally come to have such a terrible reputation?
Perhaps it starts with confusion, aided by imprecise language. There are two species of comfrey: wild comfrey, Symphytum officinale, and cultivated comfrey, Symphytum uplandica x. (The “x” means it is a hybrid, a cross.)Wild comfrey (S. off.) is a small plant–up to a meter tall–with yellow flowers. Cultivated comfrey (S. uplandica x.) is a large plant–often surpassing two meters–with blue or purple flowers.
Everyone I know grows uplandica and that is what is sold in stores. But gardeners and herbal sellers alike usually mislabel it, causing no end of confusion.
To complicate the situation even more: the roots and the leaves of comfrey contain different constituents. Comfrey roots, like most perennial roots, contain poisons. Wild comfrey (officinale) leaves have some of the same poisons. But cultivated comfrey (uplandica) leaves don't.

That clarifies it and I hope it's accurate!  I have to check to see whether the Russian comfrey leaves froze already.
Maybe I should put the lb of comfrey root powder into the compost.  Would the worms like it?
UPDATE 12/28/11:  Here's an interesting link about comfrey medicinal uses including cancer treatment translated by Google from Romanian:
Unfortunately without clinical trials, but it's interesting.