I can't vouch for the vercity of this site (and certainly won't for the spelling [rimshot]), but here's a pertinent quote:
Broomsticks were long associated wth Witches because they figured in pagan rituals, marriages and births and the Mysteries of Woman. In Rome the broom was symbol of Hecate's priestess-mid-wife, who swept the threshold of a house after each birth to remove evil spirits that might harm the child. As Hecate was also the Triple Goddess presiding over marriage, her broomstick signifies sexual union.

Old Wedding customs included jumping the broomstick, possibly to represent impregnation. Gypsy weddings always included the same ritual, though gypsies now state they don't know what it means. Oddly enough, the same broom-jumping ritual marked churchless weddings of black slaves in the nineteenth-century America.

From:http://www.geocities.com/littlepurplewitch/broomstick.html