Combine the sugar and grapefruit zest in a small, shallow bowl. Wet the rim of a martini glass with one of the grapefruit wedges, then dip the rim into the sugar mixture to coat. Garnish the rim with the remaining grapefruit wedge.
Fill a cocktail shaker with ice. Add the grapefruit juice, vodka and pomegranate juice. Shake until well chilled. Strain into the martini glass. Serve immediately.