We mustn't have tested this thoroughly enough when we were forced into rapidly deploying 10.4 on our labs of new iMac G5s, but we started having major problems with DropBoxes on our 10.3.9 AFP servers.
Network users logging in on a 10.4.x client were unable to write files to DropBoxes, although when using a 10.3.x client, there were no problems.
After tearing my hair out over it for a little while, I posted to the very useful MacEnterprise.org mailing list, and Greg Neagle came up with a solution for me.
On your AFP server, do:
serveradmin set afp:noNetworkUsers = y
This moves you from mapped permissions (where everything appears to be owned by the user and permissions granted are effective rather than real) to showing the real underlying FS perms, as long as the RecordName and UniqueID of the logged in user match those reported by the client.
Apparently this may not be an optimal solution if you have a mix of network and local users, but as all of our users are either straight up network users, or mobile accounts, this has worked seamlessly for us.