Thanks for writing back!
So, I used to have the white MacBook, which had the dedicated headphone output and the dedicated audio input. Unfortunately, it got stole from my house and now I just got a 13" MacBook Pro, which only has one "hole," which, according to the specs, acts as both output and input. I have verified that the headphones work. Check! Today, I connected the line out from my amplifier to my MBP. For a couple of minutes, the computer was able to identify the input and I was able to record (digitize) one of my albums, like I used to with my white MacBook. Then, it just stopped recognizing it, and only the built-in mic was picking up sound. I tried going to Sound in System Preferences, but I couldn't get it to work. Furthermore, I also have an iMac and I tried it there, it worked. Except for the fact that now I cannot monitor what I record as the computer records it, as I used to do. That's another problem.
So, yeah, I am all updated to Mountain Lion, and my laptop is the 13" MacBook Pro, just got it last week.
Thank you for writing!