We made baggie ice cream again for dessert tonight (it's been a warm weekend and cold ice cream tastes so good).
My new thermometer (affiliate link) arrived last week, so I thought I'd report on the temperatures of the experiment.
The rock salt and the ice mixture started out around 31.7 degrees Fahrenheit (freezing temperature of water is 32 degrees F, so the salt was already starting to work). After 10 minutes of play, the temperature of the salt/ice mixture reached 18.0 degrees F. The ice/salt mixture was so cold that it froze the condensation that was coming off of the bag! Salt does lower the temperature of ice.
I didn't measure the temperature of the ice cream mixture because I didn't want to wash the probe before (lazy, I know). I'd recommend washing your thermometer after the salt/ice mixture measurements as salt can cause damage to some materials.