This happened to my mate's 1Ds MkIII (but the crack was by the prism housing). My advice is to send it to Canon for a repair quote ASAP. My mate left it (despite having CPS Gold memership) because he was worried about the cost and the loss of use of the camera (despite owning two 1Ds MkIIs!). In the end Canon's quote for replacing the top plate (which included a full service), was pretty reasonable and they turned it around in a matter of a few days. Sadly for my friend, his woe's didn't end there; some time after, he started to notice spots appearing in his images. Even Canon have not been able to fully repair the sensor and they have told him they think that water may have got into the crack and deposited residues onto the anti-aliasing filter, which have etched into it.