I had a similar problem to you with the rear camera not focusing correctly. When you press the shutter button down half way it goes through it's focusing thing and you can see the picture sharpening but then it settles on a focal point so everything is blurred. I've taken identical pictures with a friends Nexus 4 and the difference is clear. The other problems i've had is the ffc filling with dust and the proximity sensor problems as well as the power button starting to get a little unresponsive.
I started the returns procedure on the 4/4 and described all the problems above the main subject line for the repair was "problems with the camera". I missed the first UPS pickup on the Friday (5/4) as i couldn't sit in all day and he still hadn't showed at around 15:00, and the phone was picked up on the next Monday (8/4). I followed the progress of the repair to Anovo and then after the repair it was send back to me and was delivered yesterday (16/4) on the first attempt.
The ffc had been cleaned out and the proximity sensor appears to be working but the power buttons still feels a bit dodgy and, most annoyingly of all, the camera is doing exactly the same as it was before. I got straight on the phone and my case was sent through to the complaints department who called me back this morning (17/4). They apologised and said that i'd need to send it in again for repair and that it could take another 8-10 working days. I was pretty hacked off at that but agreed, but as i'm going out to Germany next week to work for a couple of months i asked that when it was repaired or replaced that it be sent to my address in Germany. The nokia complaints guy said that he'd need to check on that and get back to me.
A few hours later he phoned back and said that the repair people had informed him that this wasn't possible and suggested that when i get to germany i go through the nokia germany website and request a repair. I checked the website and it's all in german and as i don't speak the language the prospect of sending my phone off when i get to germany doesn't appeal at all. I told him that i didn't consider this as an option and that the whole thing seemed to heading in a direction where it's easier for nokia but i end up doing their work for them. In the end I asked could i talk to a supervisor and was told that they were all busy and one would call me back. I've been waiting for the call for about 2 hours now and don't think it's going to happen.
The worst thing is that i've been an iPhone person for the last 4 years and anytime i've had a problem i've just gone to the nearest apple store (in any country) and have always left with a working device. The Lumia went wrong nearly 2 weeks ago and not only have i been without a phone for most of the time, when i do get it back it's still broken. Unfortunately companies like nokia have to realise that we're now living in a post-apple world and people expect more support that the usual 8-10 working days stuff that they they've been able to rely on in the past. I love the Lumia, but am seriously considering going back to an iPhone....
Dave
Update: After waiting for about 2 1/2 hours i called them back and was told that a supervisor wouldn't be available today, but someone would call me back tomorrow. Not impressed.