The U.S. Air Force is spending $60 million to entice 500 officers out of retirement, and back to active duty. It's the second time they've done this in the last decade, and the reason is the same; a shortage of critical skills. While the air force has no trouble (even before the recession) getting the new recruits it needs, and keeping existing people in uniform, there are always some areas where demand is high, and there are not enough qualified people. In this case, it's a need for aircrew, especially pilots, for transports and AWACs. The air force got 1,129 applicants for the 500 jobs, so they were able to be choosy.
The army has also sought retired officers to return to duty. Since most officers retire after 20-30 years of service, and can serve on active duty until 60, there are always plenty of potential candidates. Medical and physical fitness issues are the most common obstacle for those wanting to return to active service for 2-4 years. But most retired officers are in good shape.