I know this group is all about home PCB production, but I found this 30 minute video on how it's done professionally really interesting. Just thought I'd share.
http://www.youtube.com/watch?v=sIV0icM_Ujo
The thing that surprised me is that although the inner layers are done with a positive photoresist much as you might do at home, the outer layers are done in negative, then tinned and the tin is used as the resist. Quite why (and also why it's then removed) I'm not sure.
Oh - and the fact that they make up to 50 layer boards for military and avionics use!