Set up an ice bath in a large bowl - big enough to accomodate the bottom of your pot.
In a medium saucepan bring half and half, milk, sugar and vanilla bean just to a simmer over moderate heat, stirring occasionally. Remove from the heat, cover and let it steep for 20 minutes.
In a small bowl, sprinkle gelatin over water and let stand 5 minutes to soften.
Remove the lid from the pot and bring the milk mixture to a simmer once again, over medium high heat. Remove from heat, then add the bloomed gelatin, and stir until dissolved. Remove the vanilla bean and discard, or save for another use.
Dip the pot halfway into the ice bath (DO NOT LET WATER INTO THE POT), and stir continuously for about 5 - 8 minutes, until the panna cotta has cooled a bit. The mixture may start to thicken a little, this will help to distribute the vanilla bean throughout the mixture, and give it a head start to setting up.
Then, evenly divide among 6 glasses or ramekins and cool to room temperature. Transfer to the refrigerator and chill at least 4 hours or until set. *If making ahead, cover with plastic wrap.