This is what one of my desiree potato plants  did while we were on holiday in August 2012, I have never seen this before and it does look a little bizarre!
Basically the potato is in the same family as the tomato (the solanacae family) and the fruit of the potato is just like a tomato!
The only difference is that the potato fruit is poisonous and should not be eaten.
The solanacae family also includes deadly nightshade!