If you cut a longish log (say 2-3m), and leave it to season for a while, when you come to cut it up you find that the ends have dried. Keeping the ends exposed to moving air helps this process, so I imagine that if you stack the logs vertically, the bottom ends up seeming damp simply because it was on the ground and never got the chance to dry.
As Richard says, it's all about surface area and air flow. We stack our logs on bearers to keep them of the ground and cover them some of the time. The larger ones we split straight after felling, the smaller ones (say under 10cm) are left whole, but if they're birch are "striped" with the chainsaw. Striping breaks the bark and lets the moisture out - I compared some striped and unstriped birch left for 6 months over the summer, and the unstriped one was still damp in the middle of the log.
I did a little video of the way we're processing firewood now, you can see it here:
http://peplers.blogspot.com/2008/10/firewood-processing-video-and-autumn.html
This year we're stacking them to dry in similar racks, but much wider, as I've got a longer chainsaw bar now. We'll them cut them to the customer's desired length later in the summer in the racks.
Mike