1. Place first 4 ingredients in a small saucepan. Cook over medium heat 3 minutes, stirring until sugar dissolves. Remove pan from heat. Cover and let stand 1 hour. Discard rind and cinnamon.
2. Combine juice mixture and grapes in a blender; process until almost smooth. Pour mixture through a fine sieve into a bowl; discard solids.
3. Pour grape mixture into an 8-inch square glass or ceramic baking dish; skim off any foam. Cover and freeze until partially frozen (about 1 hour). Scrape with a fork, crushing any lumps. Freeze 1 hour or until completely frozen; scrape entire mixture with a fork until fluffy. Scoop into glasses, and serve immediately.