ssh-agent is used for managing lots of identifies to authentication users
against multiple computers. It seems sensible, from an ease of use point of
view, for su to become integrated with ssh-agent.

Example:
# give ssh-agent access to my private key
ssh-add
Password: (once)
# Authenticate against remote server
ssh user@remotebox1
# Authenticate against remote server 2
ssh user2@remotebox2

# Change to root
su
Password: (every time)

It would be good if I could give su a list of trusted ssh keys.