As Eagle Dynamics evaluated the concept of doing a helicopter simulation with a 6 DOF clickable cockpit, new terrain area, very detailed avionics and a new mission and campaign structure, it became clear that they could not use the Lock On code, and would have to develop a highly-modified version of their existing 'The Fighter Collection Simulation Engine' (TFCSE) with new code. Additionally, whereas Lock On was a survey simulation that featured several aircraft at a medium level of fidelity, they wanted to start a new product line that studies one aircraft at a time in exquisite detail.
Taken from the FAQ on: The Digital Combat Simulator Website.
This led to the construction of what Eagle Dynamics calls the "Digital Combat Simulator series", a virtual world in which different games can all co-operate with or fight each other in network multiplayer mode. However, each and every game (modelling only one aircraft or helicopter) can be played stand-alone.
So far, as of Oct 2010, DCS: Black Shark is released and A-10C Warthog is announced. Besides that, Eagle Dynamics (in that same FAQ) states they expect to be busy releasing new plane- and helicopter-sims in that same series for the next decade.