Never grown it myself, but here's what I have on it (not much!). The information relates to Pimenta dioica syn. P. officinalis, but you probably have one of the following:

Calycanthus floridus, Carolina allspice
Calycanthaceae C. sterilis
Calycanthus floridus glaucus Eastern sweetshrub; Carolina Allspice
Calycanthus occidentalis

You can get more information about these at this site:
http://www.ibiblio.org/pfaf/cgi-bin/find_lat?LAT=&COM=allspice&FAM=&RATING=1

Description: Perennial, evergreen tree to 7-13 metres. Smooth grey bark, with elliptic, glossy leaves, dark green, up to 15cm long. It has small white flowers appearing in mid summer, followed by green berries that turn purple when ripe. Sometimes called Pimento.

Fruit starts to develop after about five years, and becomes full-bearing after twenty years. Male and female trees are required.

Berries are picked when they have reached full size, but before they can ripen. Dried allspice berries resemble large brown peppercorns. Unripe berries are harvested and sun dried until the seeds in them rattle. They vary in size between 4-7mm in diameter and are dark brown with wrinkled skins. The outer case contains two dark, hard, kidney-shaped seeds. Whole dried allspice will keep indefinitely when kept out of light in airtight jars. It can be ground in a spice mill or an electric coffee grinder. The ground spice loses flavour quickly. Leaves from the male trees are also harvested for eugenol oil.