If you are declining enrollment for yourself or your dependents (including your spouse) because of other health insurance or group health plan coverage, you may be able to enroll yourself and your dependents in this plan if your or your dependents lose eligibility for that other coverage (or if the employer stops contributing towards your or your dependents’ other coverage).
Medicare 2012 Handbook Medicare 2011 Handbook Medicare 2010 Handbook Medicare - Who Pays First