home_url() function is implemented in WordPress since version 3.0.0, and you are using version 2.8.5.
To fix this error edit wp-content/themes/twentyten 3/header.php file –
replace:
home_url();
with:
'http://'.$_SERVER["SERVER_NAME"]
This is hotfix, and i’m afraid you will receive more errors like this.
“Responsive Twenty Ten’ are compatible with 2.8+” – as you see it’s not true 🙂
Best solution is revert theme to old one, or install newest wordpress version.
EDIT
It would be better to use get_bloginfo( 'url' ) than $_SERVER data.