You can use some actions and filters on wordpress you can use to customize your login page:
Here are some i found in the wp-login.php file
action login_head
, to output html before the header tag is closed
filter login_headerurl
to customize the link where the logo goes
filter login_headertitle
to customize the title of the logo
filter login_message
to customize the error messages
filter wp_admin_css
to edit the css used by the admin
action login_footer
to add any htmk in the bodytag is closed
The logo comes from the css rule h1 a
All hooks and actions can be added on your theme functions.php or within a plugin.