Reply #2: You have to enroll, I'm pretty sure.

[SheilaT]

They don't just send you out your Medicare card.

Also, Medicare is primary except in certain specific instances:
You are receiving Black Lung benefits
You have Group Health Coverage of your own or through your spouse. If through your own current employment, your employer must have at least 20 employees. If through your spouse's employment, that person's employer must have at least 100 people on the payroll.
The VA (Veteran's Administration) has authorized and agree to pay for specific treatment.
The illness or injury was due to a work related accident or condition, or a non work related accident for which there is some kind of liability coverage.

I do out patient admissions at a hospital, so I know the MSP (Medicare as Secondary Payer) questions by heart.