They are bots. I've written a plugin to deal with them. but it's currently not working with WP-2.5.
And to be honest, the best way to deal with them is to rename/edit all the files that use the fields used for the real registration submissions, but leave user_login or one of the default fields in tact in the form, but hidden by CSS. You can then check to see if the hidden field was used.
A bot will see the hidden field.
Those that do, are sent off elsewhere.
For instance, here is a recent series of post submissions to my own registration form:
user_login = shoneetek
user_email = helga_krasavitsa@mail.ru
submit = Register
action = register
78.157.143.200
I check to see if user_login is filled in. If it is, they are scurried away to another site, never to be seen again. I get no email, and they dont get to register.
That's confusing to you prolly, but its very effective.