After reading the posts here, I just, carefully, took apart one that I have retired. To me, the rubbery thing at the bottom is definitely a seal. It seems to be there for two things - 1. to insulate the center battery post, and 2. to seal the liquid chamber at the bottom. It is actually more of a grommet with a small flange on the top, just under the atomizer, and a larger flange on the bottom under the battery post.
The one I took apart was leaking at the bottom, a definite leak and not just condensation. Examining the wick cup carefully showed an "open" space, quite small but there never the less, where I think liquid was able to leak. It might have been repairable by carefully relocating the exact position of the wick/coil assembly in the cup to center it better. It was definiterely off to one side of the cup. That could be the basic problem when leaks occur, the coil/wick assembly is not properly blocking the air hole at the base.
As a side note, I also found out where that piece of wire comes from that seems to stick out sometimes in the vicinity of the wicks. It is one side of the coil connection. That side is made by wrapping the coil lead up the side of the cup befor it is press fitted into the metal base. If it's a little long it will stick out and can actually be trimmed with a pair of flush cutters if you wanted to.