How to get Mac OS X Terminal.app and screen/vim scrolling to play nice?
In Lion, the default terminal type is different, so you should use this line in your .screenrc instead:
termcapinfo xterm-256color|xterm-color|xterm|xterms|xs|rxvt ti@:te@
In your home directory, put in the file .screenrc the following text:
# Make xterm scrolling work properly with screen.
termcapinfo xterm-color|xterm|xterms|xs|rxvt ti@:te@
Works for Terminal.app too.