To get going: If you have a marketing-driven social media plan, that's a start. (If not, see Time's person of the year award for 2010.) Now layer in content that attracts potential candidates--company news, color commentary from employees, video of the work environment and of course job information.
The Veterans Affairs Department, perhaps the largest federal agency to make a big push for mobile devices, has settled on security at the application layer, a course that the largest potential federal buyer, the Defense Department, is unlikely to copy.