I've got both machines in my group, and must say if I was buying new, I would definitely get the iBook. People say the new case is "cheap", but I find it feels very robust, and looks a little less "girly" now, since the interior is almost grey, not shiny or white. I'm *extremely* impressed by that machine for the money.
The 12" PB is pretty, and if you can get one in good shape for the same money, then sure. For me, the 12" screen is too small, so I have the 15" AluBook.