Is there any difference (e.g. regionality) between the two forms of the past participle of melt (melted and molten)?
They're not really alternatives. Molten specifically refers to liquids which are extremely hot, and whose usual form is as a solid. It is used as an adjective.
By contrast, something may be melted (whether used as a transitive verb, or an adjective), but need not be hot, and it need not be liquid (something that has begun to lose solidity can be considered melted), nor does it even need to still be subject to the melting process (i.e. it may have fully resolidified).
Marcin's answer is, I believe, very good, from the perspective of every day usage. However both terms relate to concepts in physics, so let me give some more perspective.
If you look in wikipedia entry for molten/melting you'll find this distincion:
This is further enforced by dictionary definitions, but also expanded with what used to be colloquial use,
and for melt
From these definition you can conclude that
So, you should make a distinction between a strict physical meaning and colloquial use. I believe that discrepancies due to loose vs strict meaning are much more pronounced than any distinction due to regional meanings.