Disclaimer:This is speculation, like all Cataclysm gold making guides there is a risk you may not profit as much.
With Cataclysm around the corner (3-4 months if they want the projected release date of 2010) we have found many new guild achievements. One achievement involves getting legendary items. There is profit to be made from this.
Below is a list of materials needed for the Legendary Ragnaros hammer along with their current price (server dependant).
Fiery Core - Item - World of Warcraft - (Fiery Core) - AH price 20-50g each - Resell in Cataclysm for 100-150g
Lava Core - Item - World of Warcraft - (Lava Core) - AH Price 30-70g each - Resell in Cataclysm for 120-200g
Dark Iron Ore - Item - World of Warcraft - (Dark Iron Ore) - AH Price 10-30g per stack - Resell in Cata 60-90. If you smelt them to Dark Iron Bar, Resell in Cata 100-120g
Once again this is all speculation.