I have two seedlings from the seeds in a store bought grapefruit.
I have grafted one of them with material that I got from a forum member in Europe so I hope that it didn't contain any pests.
I'll link to the images since they are big.
It's not water on the leaves, it's something sticky which doesn't dry.
First plant, only visible on one leaf:
http://i50.tinypic.com/vex93n.jpg
Second plant, many leaves and the stem are affected:
http://i45.tinypic.com/29l0lmc.jpg