i have the same problem with w/my tomatoes. i think it's called 'blossom end rot.' see below for more detail.

also, to avoid future rotting i am thining about picking all the green tomoatoes NOW and letting them ripen in a window sill? is this a bad idea??

BLOSSUM END ROT
Symptoms:
Blossom end rot can affect fruit at all stages of development. The characteristic symptom is a progressive deterioration of the blossom end of the fruit, from a water-soaked appearance to a sunken, black, leathery lesion. While secondary fungal infections may occur, blossom end rot is a physiological disorder caused by calcium deficiency induced by water stress.
Control:
Careful water management practices are a key. Irrigation and/or mulching are important. Application of lime or calcium prior to planting may help. Liquid fertilization using calcium nitrate can be used for small plots.

many thanks becca -- will look into this...