Canon has issued a service notice for the original Canon 5D.  This is not for the Canon 5D Mark II.

From Canon:

We have discovered that, in rare instances, the main mirror of some EOS 5D Digital SLR cameras may detach due to deterioration in the strength of the adhesive. Accordingly, we would like to convey the details and our service policy concerning this phenomenon.

We offer our sincerest apologies to those customers who have been inconvenienced by this issue. Canon always strives to provide the highest quality products to our customers and we will spare no effort in our quality management to make sure our customers can use our products with confidence. We hope our efforts will earn your understanding.

Phenomenon
The main mirror of the camera detaches and images cannot be viewed through the viewfinder.

Affected products
EOS 5D Digital SLR cameras whose main mirror has detached.

User Support
We will repair and reinforce the mirror portion of the affected products free of charge. If you own one of the affected products, please contact our Customer Support Center.

More info from Canon.