Deepholm is a decent mining zone, even after the initial nerf at the beginning of Cataclysm that removed over half of the mining nodes.
The entire zone is made out of rock, so, nodes can be found just about anywhere. The blue line in the map shows an optimal path for mining Elementium and Obsidium in Deepholm for hitting as many nodes as possible, though, more than any other zone, you shouldn't be afraid to veer off the line. Nodes are literally everywhere. Basically, you are looking for fissures, pinnacles (nodes are often found at the base of tall narrow rock formations), or elevation change. The zone has three "tiers". The lowest tier, a middle tier, and a high tier. A lot of the ore is found in the transition between these tiers, which is why it makes sense to fly in a circular pattern around the zone. The other reason you want to fly in a circular pattern is that the elevation change is significant between each tier, and just from an efficiency standpoint, you don't want to be having to change elevation too often.
Another nice feature of the zone is that it is the only zone that you can mine both Obsidium and Elementium ore. You can even find Rich Obsidium Ore nodes here, which can produce double the amount of raw material as a normal vein. The drawback to the zone is that the Elementium nodes can't produce Pyrite Ore. How much gold can you expect to add to your World of Warcraft gold pile farming in Deepholm? It depends. It is always a good idea to know how much your raw materials will fetch on the Auction House before you start a farming expedition.
If you also happen to be versed in the Herbalism profession, Deepholm possesses a unique herb: Heartblossom. Currently, it is second only to Twilight Jasmine in terms of value on the Auction House, which is of course subject to change as the Cataclysm economy matures. Farming any of the Cataclysm zones with both Herbalism and Mining will lead to a lot of WoW gold in your account.
Mount Hyjal Mining Route
Vashj'ir Mining Route
Deepholm Mining Route
Uldum Mining Route
Twilight Highlands Mining Route
Tol Barad Mining Route
Mount Hyjal Herbalism Route
Vashj'ir Azshara's Veil and Stormvine Route
Deepholm Heartblossom Route
Uldum Whiptail Route
Twilight Highlands Twilight Jasmine Route