Hi Rob, yup Marc's got the answer! (hold down shift+ctrl).

Just one additional note on that - if you hold down shift+ctrl for the initial click it will force the marquee to happen, and then after your initial click if you want to deselect rather than select the things inside the marquee, release just the shift key so that only ctrl is held down and that will switch it to deselect mode. A little sleight of hand for that one with both keys for the initial click and then keeping just one down while dragging the marquee.

Similarly on a regular marquee where you just click in an empty area to start it, if you hold down Ctrl it will deselect targeted objects rather than select them.

- Michael