Obsidium ore is the new starting area ore for the level 80-85 zones that were added in the Cataclysm expansion.
Obsidium is to Cataclysm what Cobalt was to Wrath of the Lich King.
This means that Obsidium will be found in the starting zones of the expansion and fade off by the middle range zones.
The best places will be the level 80-82 zones which include: Mount Hyjal, Vashj'ir, and there may be traces in Deepholm.
To start mining Obsidium will only require a skill level of 425 which is 25 less than the max of 450 in the Wrath of the Lich King.
This will allow players that are 425+ to skip finishing off their mining to 450 in Northrend, which will become an old expansion on December 7th.
Vashj'ir is a new underwater zone and players will have to complete several quests at the start in order to get a Seahorse mount and underwater breathing so they can effectively level and go through the zone.
Anyone seeking some underwater mining enjoyment or lack of, should go there or avoid it as they please.
This zone will have a lot of Obsidium, but players might not enjoy underwater mining.
The best option of these zones is probably Mount Hyjal because it is located on the world map and does not require a pre-quest to go to the zone, although, without the pre-quest players will not be able to quest.
For mining this works well though, simply fly south from Winterspring and into the zone and begin mining.
This is also the recommended zone to start in for players driving to get 525 mining as quickly as possible.