I think we need to figure out if it's Skype that's the problem, or the driver. I've found a few posts about Skype volume on the Mac in general with people complaining about the auto gain control and there's a tweak to disable it. I have a unibody Macbook and I've never noticed any issue though..
EDIT - Ok, I've tested with Audacity and it's also way too quiet, however with the Mic sensitivity on max in the Sound System Preference Pane it's just about ok. Skype's still unusable though. I'll try turning off Skype's auto gain control.