I have been following this issue for a few weeks and as I understand it, Olympus has confirmed that even a fairly brief exposure of the EVF eyepiece to direct sun will result in damage to the EVF in the form of one or more colored spots or “blobs” visible within the viewfinder. It is only correctable by sending the camera in for repair.
It does not show up in images nor is the monitor screen susceptible I guess, but I am of the impression that this is not necessarily limited to just the E-M1.
So you may wish to take precautions when carrying or leaving the EM-1 anyplace where the viewfinder may be exposed to direct sun.
I have no idea if this issue is mitigated by the use of a housing, but it would seem unlikely. I don't know whether it is affected by either the camera or the viewfinder being on or off. I suspect it is not an inexpensive repair.
Here are a couple of links if you want to read more:
http://www.dpreview.com/forums/post/53792415
http://www.mu-43.com/showthread.php?t=64334
http://www.dpreview.com/forums/post/52857656
Disclaimer: I am repeating what I have read elsewhere; I have no experience with this issue and have not had any communications with Olympus.