Turtling in Starcraft II will be somewhat possible for the Terran race, but more difficult for other races than in the original Starcraft. This is what Karune reported on the Battle.net forums:
Karune: “Overall, ‘turtling’ is a bit harder in StarCraft II since every race has new ways to keep them mobile. Terrans will likely have more Dropships than before, since they double up as medics. Zerg has the Nydus Worm, allowing whole armies to be transported to a set location. Lastly, Protoss has ‘Warp-In’ along with the newest version of the Mothership, which is slower, more hardy (in terms of hit points), and allows for the player to teleport a whole army to its location.
Nonetheless, looking at the three races, Terrans are still relatively the best for a ‘turtling’ strategy. In the current build, Terran Siege Tanks’s AoE (Area of Effect) attack have 13 range (more than twice the range of a Marine) against ground units, and Thors’ AoE attack have 10 range. Getting an army of both is quite expensive- though if you can build the momentum, it is quite difficult to stop this ranged combination.
– Units under Dark Swarm do not take damage from Snipe in the current build of the game. (Source)
– Creep damage does not currently stack. At the moment, creep does 3 damage per second to an enemy structure. The rate of damage is of course subject to further balance. Creep will not do damage to enemy Zerg buildings. (Source)