Unfortunately, I have tried to use Leopard as an Open Directory Master, Calendar Server, and AFP server at one account since 10.5.2 was released and in stops functioning under very light load at least once a week. The server starts refusing connections and complains and incorrect user:password combination was entered. After troubleshooting, I have not determined the cause, but I did find out the problem can be solved by stopping and starting the AFP server. Hopefully this will be fixed with the 10.5.3 release.
I have been having this same AFP problem with Leopard Server. The issue is unfortunately epidemic. I was myself hoping it was going to be fixed in 10.5.2. I have had to convert one graphics lab's home folders to NFS (not secure), and they have to connect to file shares over [shudder] SMB! Blech.
I finally, however, discovered this: If you first install 10.5 Server with the "Simple" settings, even though it doesn't initially turn on many of the features one might eventually want, it does do some automated configuration that a) takes some of the nitty-gritty hassle out of setup, and b) seems to make AFP work right!
After you've done Simple, you can config and test the basic features using the new Server Preferences, and then eventually go into Server Admin and Workgroup Manager to get more detailed.
Now, admittedly, the AFP bug is still a stupid thing for Apple not to have fixed yet, or provided a kbase article to solve. And the solution I've just described might not work for every environment. But I was at least heartened that AFP will in fact work on 10.5 Server, and I got to glean some Apple-sanctioned configurations for other services, too.