I've had this issue for a while now, and I'm running the latest version of snow leopard on my 15'' macbook pro early 2011. When I start up my computer I would have this problem where if I don't have my ethernet cable plugged in when it first starts up it won't be recognized. So if I go to system preferences and go to network it'll say "not connected" even though my ethernet cable is connected.
This problem would only resolve if I restart the computer (I've tried logging out and into another account, etc. but it still didn't work). I've also tried using the diagnostics assistant but it wouldn't help. This problem would also occur say if I unplugged my ethernet cable for a bit and plugged it back in. The computer would say it is still disconnected even though it is not. During this whole time I would have no problem accessing the internet from wifi, unless I turn it off (then I would have no internet connection even with the ethernet cable plugged in). I'm also wondering whether this is a software/hardware problem or has to do with the internet modem.