Leaving aside all the Ruby, Rails, authlogic, and so forth
questions... I advise you not to do this. It means you're using email
addresses as the primary person-identifier (as opposed to ID in the
database, which I ass-u-me will be numeric). That's perfectly fine
from a technical point of view, but not for user convenience.
People often have multiple email addresses. People change email
addresses, for numerous reasons. People leave sites for a long time,
try to come back, and forget what email address they used when they
signed up. Been there, done that, couldn't log in to get the T-shirt.
But they usually have a preferred username. (Maybe two, one for
places that insist on "real" names and one for everything else. Plus
maybe variations, if someone beat them to their preferred name, or
some benighted site insists on very short usernames.) They may go
away for a while and come back, and their "old" user ID will be
obvious to them, at least after a small number of tries.
Also, before people know your site well enough to trust it, they may
be reluctant to reveal an email address. They will probably not,
however, have any problems telling you what username they want to use.