Original camera had a fuzzy dark spot in images. Wasn't on the back plate lens, or on the outer camera lens. Figured it must be inside the camera or somehow a defect on the CCD from age/wear/drops. Decided to replace whole camera with new from iFixIt
Went smoothly. Guide on iFixit was complete and had good warnings about keeping metal-metal contacts clean, and keeping proper track of location of screws as you remove them.
Got down to the camera and pulled it out, and put the new one in... Noticed as I put camera connector back on that it didn't "click" quite cleanly... seemed "mushy" as I pushed down with my spudger. Adjusted the placement a little, and tried again, but no solid click. But it did stay down, and it was generally in the right place, so figured it was okay. Guide seemed to show (as did online videos from others...) that there was really no way to misalign this. Put the phone back together, but it *wouldn't* power up or restart!! No Apple logo at all.
Took it apart again, checked everything... still no joy. Did this a couple of times, slowly getting worried I'd "bricked" the phone. Did the standard Apple recommended resets. No joy. Did a hard reset (had a good backup already). No joy. Finally, took the phone apart down to the camera again, and looked very carefully from the side at the connector.
I could see that the way I was putting it on, it was missing the alignment a bit, even though from the top it looked just like the old camera connector's placement. So, this time I watched from the side as I carefully placed the connector down... this time getting aligned properly. It gave a much more solid "click"... I put the phone back together and it started fine.
New camera fixed the weird dark spot, and my 3-year old iPhone 4 lives another day, hopefully to last another year or so until the next gen after the 5 comes out.
Watch very carefully the alignment of connectors as you replace them in reassembling your phone, especially ones that weren't factory done (like my new camera), as they may fit differently, and haven't "adjusted" their cable shape the same way the factory ones have. Otherwise, the repair was quite easy, and should be able to be accomplished by just about anyone.