If you have a rather robust ssh config file, add the following to the bottom of your file:
# Fix Leopard Issue with Null Replacement
You can also use id_dsa if that's your preference.
To generate a id_rsa/id_dsa file, use ssh-keygen.
Check the man file, 'man ssh-keygen', for the details.
Is something going on with SSH on Leopard? I never had to use the "-i" flag before today, and suddenly I can't connect without it (or the IdentityFile directive). A few days ago, I had to add DISPLAY to my bash profile in order to start remote X sessions, when I never needed to before (the system suddenly started forwarding Leopard's '/tmp/' launch path instead of a DISPLAY IP). I haven't changed any system settings or (knowingly) updated any Mac or SSH software in the past week.