Purphoros will stop being a creature and will be removed from combat.
At the moment, I assume that, due to Purphoros no longer being a creature, he will not deal damage.
Once the Theros FAQ has been released, we will be able to answer this question without guesswork.
The FAQ has been released. You can access it here: http://www.wizards.com/magic/magazine/article.aspx?x=mtg/faq/ths
and I just copied what it said.
@Denial048: The Theros FAQ is not necessary in this case. The Gods have static abilities that cause them to change types depending on the game state. If your devotion to red ever drops below five, Purphoros, God of the Forge will stop being a creature.
Also, devotion requires you to have mana symbols in the mana costs of permanents you control. This is different from requiring mana, which would make the Gods function like Figure of Destiny .