That's exactly how it works, although the second touch to toggle has to happen over the area where the check appears, so the area is quite small. The "gesture" to toggle the check therefore is to swipe from the left (the N900's bezel) to the middle, which will toggle the check (only if the item is selected). This should prevent accidentally checking/unchecking an item.

This also works in non-fullscreen mode should you prefer to touch the items directly instead of using the toolbar button.