From the experts:
British-born novelist Plum Sykes has taken this rich, fruity name out of the produce section and put it into the baby name basket. It's more appealing than Apple, more presentable than Peaches. The French equivalent, Prune, is very fashionable there but would not fly with English speakers.
Plum Sykes was born Victoria, and her nickname derived from the variety of fruit called the Victoria Plum.
Plum makes an appealing middle name choice--Moon Unit Zappa used it as such for her daughter Matilda.