Well as a guy who has run into this problem before with the same in/out comes I would say it's worth a shot. At the very least try to reseat the connections or reverse the plug and see what you get.  We took down a clients 7-55 last month, which we had not done an update to since intial delivery in May, so that they could renevate. When we came back for re-install last week the issue came back up with the known good 1.4v cables. We then swapped them again for new ones and did the most recent update of the ATV and we have no problem (fingers crossed). I know it doesn't make sense, a lot of our problems in B&O don't but they still happen :).  I have posted on this in the past, if you can find the true culpret I would love to know what it is, but since the exact symptoms can be seen on other manufacturer's products I can't genuinely say that it's a only B&O problem..... 

@tIRED I hate this for you.  At least use component cables so you can enjoy the ATV until it is sorted out.  Given the source, there is certainly no loss in quality.  BTW, depending on your agreement with your dealer regarding the software update, perhaps they could give you a little assistance?  Learning lesson I won't forget; If you pay for a service visit, have them take the extra 4 minutes and roll through all all of your features before leaving.    

I'll give the cables another shot this weekend. Will try to decode the HDMI connection with a HDMI analyzer from work. Not really used to dig into the HDCP stuff but hopefully I can learn as I go.. :)

Just to report back - new cables (1.4v) with absolutely no difference. No support form B&O store (they simply don't understand and do not get support from B&O). "Get another media box that isn't AppleTV" is their suggestion.. Fantastic.. 

I experience similar problems as tIREd, i.e.  "Warning - Content is protected - Select another source" with a brand new Mac Mini connected with a HDMI cable v1.3b to my BS3 with SW 9.01.   I am considering buying HDMI v1.4 cables, but understand this will not solve this issue;

@BeoMegaMan - do you recommend any specific brand of HDMI cables?  Has anyone been succesful in setting this up?  

Hmmm, if your going to try the cable swap then make sure source them from a shop you can take them back to if it doesn't do the trick. :) 

I swapped the HDMI v1.3 for a v1.4 cable but no change: I can see ~5 seconds of the start-up process of the Mac Mini after which the screen turns black and the message "Warning - Content is protected - Select another source" appears.

I can upgrade the BS3 SW, from v.9.01 to the latest version (10.3?), but given the subject of this thread I doubt this will resolve this issue.

any other hints? 

@lowroad and tired, try going into the system menu (red+go) and select picture adjustments and put your framelock to double buffer. Power down and try it again.... just throwing out ideas.