[Note that I am a sysadmin, but this seemed more like a question for superuser than for serverfault].
I have a Apple iMac Intel (Early 2008) and can not insert a Cat-5 cable into the ethernet jack. [I recall that, when we initially set up these sorts of computers, I had to force the cable in on some of them.]
Upon inspection, it appears that there are two springs inside the port that physically block access for inserting the cable. I compared against another unit of the same type, and the springs were not blocking the cable there. I can move one of the springs with a small screwdriver bit, but haven't been able to keep them out of the way.
And yes, the computer is off warranty.