Solution:
Nails don't hold because the gutters are constantly expanding and contracting. However, a good glob of exterior epoxy on the bottom two inches of the nail will hold everything together. I use West System epoxy for a lot of exterior work and it holds up very well. Also if using epoxy and nails, drive the nail in enough to barely touch the gutter, since any tighter can cause the nail to move or the gutter to get dented when it expands.
You have a couple of other choices. Gutter screws are available at most lumber yards. They look like giant drywall screws and do a good job. I have also used "Timberlok" screws, which are made to hold landscape timbers together. -- Richard, on the OHW Bulletin Boards.
And for a full primer on gutters, see this story.
