The first part of my Automatic Backups with cron, tar, and SSH details how to set up remote login (sans passwords) using RSA public key.
Apparently this doesn't work if your home directory allows groups or others write access. So if used ssh-keygen -t rsa to generate a public key which you added to a remote host (under .ssh/authorized_keys), but ssh is still prompting you for a password, try setting your home directory's permissions to something like 0755 (or remove write permissions from other and groups)!
