This plugin is for single site installs, it will change the link and title from the WordPress one to your own site.
Custom Login Page Link
This is the link and link title on the login page.
I posted a small plugin over at WPMUDev for users, here it is again:
along with some code snippets on how to handle this:
http://premium.wpmudev.org/forums/topic/remove-wp-about-menu-from-toolbar#post-161711
There are a few options. First the manual edits:
Open the file:
/wp-includes/class-wp-admin-bar.php
And around line 456:
456
| add_action( 'admin_bar_menu', 'wp_admin_bar_wp_menu', 10 ); |
Comment it out or just remove the whole line. Now the WP links part is all gone! 
But….. But……… If you wanted to edit those links to customise yourself, you could just go open up the file:
/wp-includes/admin-bar.php
You’ll see it all near the top, here is a short snippet:
if ( is_user_logged_in() ) {
// Add "About WordPress" link $wp_admin_bar->add_menu( array('parent' => 'wp-logo', 'id' => 'about', 'title' => __('About WordPress'), 'href' => admin_url('about.php'), ) );
}
You will note much of the same on there.
The logo is part of a sprite which can be found here:
/wp-includes/images/admin-bar-sprite.png
Both of these options mean updating the code every time you upgrade your WP install.
And now the final option, which is the easiest, but its yet another plugin………
Remove WordPress Admin Bar Links
Enjoy, its free!!