(I fell asleep at 9:30 last night, so I'm sorry I didn't post this sooner!)
Here's the science behind brining: the salt water tenderizes the breast meat, while the rest of the chicken cooks. What usually happens with a whole chicken or turkey is that the white meat gets done before the dark meat so that the white meats ends up tasting dried out as you keep the chicken in to cook the dark meat. The brine keeps that from happening, and you can get the real details from Alton Brown's awesome cookbook: I'm Just Here for the Food (I was brining poultry way before he was talking about it, though!)
Julie's Brined Roasting Chicken
8 cups warm water
1/2 c kosher salt (must be kosher salt!)
1/4 c packed brown sugar
3 T. molasses
1 T. whole peppercorns, crushed (or 1 tsp. pepper in a pinch!)
1 T. whole allspice, crushed (or 1 tsp. ground)
2 tsp. ground ginger
4 lbs. roasting chicken
4 cups cold water
For the brine: combine first 7 ingredients in a large pot. Bring to a boil, cook and stir until everything's dissolved. Remove from heat, cool to room temperature. Remove giblets from chicken, discard. Add chicken to the pot, breast side down. Refrigerate for at least 3 hours or overnight.
When ready to roast, remove chicken from brine and discard brine. Place in roasting pan and roast for 80-9- minutes at 350 or until chicken registers 180 in the breast meat (tent with foil if it's getting too brown). Usually, you can make great gravy from the drippings, too.
I do this to my Thanksgiving turkey every year, as well. I use an apple cider brine and put the bird and brine in double thickness turkey oven bags set in a large roasting pan. I did two turkeys for our first church thanksgiving dinner and everyone was pleased with how moist and delicious it was. Even my red-meat loving brother in law lets me do the turkey when we eat at his house.