I've got public key authentication working between my Mac OS X and an Amazon EC2 instance so that from the command-line I can just type the following and it works:

ssh root@[IPAddressOfEC2Instance]

The strange thing is that I can't seem to mount the instance using "Connect to Server" in the Finder.

I've tried typing the following server addresses into the "Connect to Server" dialog:

ftps://[IPAddressOfEC2Instance]
ftps://root@[IPAddressOfEC2Instance]

But all I get is

You entered an invalid username or password. Please try again.

The root user on the EC2 instance has a blank password and I'm wondering if it has to do with that. However, I can't change the password for the root user.

I can use an SFTP client to connect to the machine, I just can't mount it with "Connect to server". It asks for a username and password (for a registered user) and it's root/[blank] which it doesn't accept. The other option is "Guest" which brings up an empty folder in the Finder.