Password dialog appears for password-less SSH private key
I had this same problem.  However, when a generated a new password-less private key, using the following command:

ssh-keygen -b 1024 -t rsa -f id_rsa -P ""


I no longer saw the password prompt.

Additionally, ssh-add failed to add the old key, but added the new one as expected.

I generated the old key on Leopard in 2009, using what ever version of OpenSSL I had grabbed, built and installed back then (that Mac died, so I can't log in and check what I was running).  Something about that key was incompatible with Lion's native SSL libraries.

I backed up my old key, so if anyone wants to suggest some checks, to identify the key's specific properties, let me what to check and I'll report back.

Another clue - I noticed that my old id_rsa.pub file had extended attributes.  i.e. it's permissions flags looked like this r--------@ instead of r--------

xattr -l id_rsa.pub.old


returned:

com.macromates.caret: {
    column = 0;
    line = 1;
}


cruft left over from TextMate.  I don't know if removing it would have fixed the issue without my having to replace the key. I think it's unlikely.  

In case you (future reader) are seeing the same thing, you can remove the extended attribute as follows:

 xattr -d com.macromates.caret id_rsa.pub.old


You can stop TextMate from adding them by first exiting TextMate and then issuing this command: 

defaults write com.macromates.textmate OakDocumentDisableFSMetaData 1