I am having a strange issue with iTerm2, in terminal vim (non-gui) and the solarized color scheme. First, I have set iTerm2 to use the dark solarized colour scheme.
This worked for me in OS X 10.9.1 in iTerm 2 as well as Terminal. One mistake I was making was putting the colorscheme declaration before the termtrans and termcolors settings (and I needed both of these to make it work). As others have said, I set my terminal type to xterm-256color
if !has("gui_running")
let g:solarized_termtrans=1
let g:solarized_termcolors=256
endif
colorscheme solarized
set background=dark