you might have broken the latch. it's a little plastic part on the top edge. It might still be there but cracked or something. If so, it should be covered under warranty - I had them replace my entire logic board for this once (on an ibook g4). Not that I wanted an entirely new board, but the parts aren't separate.
oh, i didnt break it off that i can remember. first time i used the ethernet cable at all was when i started going abroad (about two weeks ago).
Going to be out of the country until the middle of january. For the most part, I use wifi anyways and if I ever use ethernet (only now), it's usually when it's stationary on the table.
In this instance i've been using it in bed and such. oh well. i'll just have to be careful.
The little plastic tab on ethernet jacks on the cables can easily break off...and this "sliding out" of the cable can easily happen if this piece has broken off the connector. I have many cables with broken tabs on them, they work, but you have to be careful if you move a computer because the cable can easily slide out.