How do I setup 1Password on a second machine to use a shared Agile Keychain file in Dropbox?
I had this exact problem over and over, with some help from Agile I found the answer here: http://help.agile.ws/1Password3/configure_dropbox_on_mac.html
Here is the important part:

To configure 1Password on your other Macs, first setup Dropbox, then
make sure you have the latest version of 1Password installed and
running.
Locate your 1Password.agilekeychain file in your Dropbox
folder. If it has a green check mark badge indicating that the sync is
complete, double click it,
and 1Password will confirm that you want to
use this new location. Important If it shows the blue syncing badge
rather than the green check mark badge, wait until the badge is a
green check mark before opening the file. Failure to do so could
result in data corruption.