I keep it in single line, if it's short. Lately I've been using this style for longer or nested ternary operator expressions. A contrived example:
$value = ( $a == $b )
? 'true value # 1'
: ( $a == $c )
? 'true value # 2'
: 'false value';
Personally which style you use, or find most readable?
Edit: (on when to use ternary-operator)
I usually avoid using more than 2 levels deep ternary operator. I tend prefer 2 levels deep ternary operator over 2 level if-else, when I'm echoing variables in PHP template scripts.