Mount Hyjal is probably the easiest zone to farm Obsidium Ore if there aren't too many other players farming it.
The highest concentration of the ore is found in the blighted area to the southeast (right of the black line on the map). The ore is typically found where the hills meet the flat land. Though the south is certainly the best area, you can extend your mining route to other hilly areas of the map. The optimal mining path for Obsidium Ore is represented by the blue line on the diagram.
Because Mt. Hyjal is the best zone to farm Obsdium Ore in the Cataclysm expansion, you'd do well to spend some time here if Obsidium follows the same path that its Wrath of the Lich King expansion counterpart Cobalt Ore did. Cobalt Ore held at a higher price than Saronite Ore (Catacylsm's Elementium Ore equivalent) through most of the expansion cycle because farmers were after the rare Titanium Nodes. Whether Pyrite Ore lives up to the prices that Titanium Ore saw throughout much of the expansion will only be known in hindsight, but there is no reason to suspect World of Warcraft game designers will change what works.
If you also happen to have herbalism as a profession, you can find many Cinderbloom spawns in the blighted area of Mount Hyjal and many Stormvine nodes in the the lush grassy area. While neither herb is unique to the zone, in combination with mining, you can make a lot of gold for your World of Warcraft piggy bank.
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