I have tried three different microphones with my MacBook unibody and none of them worked.
Yes I did select them as the audio input from the system preferences :)
Now, I've read here and there that the line-out does have some problems with some headphones, so I was wondering if the line-in suffers from similar problems (and I couldn't find anyone talking about that).
Since I've also read the line-in problems are mostly caused by poor headphones, it might help you to know that the last mic I've tried is new, and while it's not an expensive one, it's not the chepest either (paid 12€, when the cheapest, from the same brand, costed about 5€)