1. Stir together all ingredients and a pinch of salt in a heavy medium saucepan and cook, covered, over medium heat, stirring occasionally, until plums fall apart, about 25 minutes.
2. Discard cinnamon stick and zest.
3. Purée in batches in a blender until very smooth. Cool, uncovered, then chill, covered, until cold, at least 2 hours.
4. Freeze purée in ice cream maker, then transfer sorbet to an airtight container and put in freezer to harden, at least 1 hour.