Framework should not call check_plain on already escaped variables

variables like $site_name and $site_logo are already escaped by Drupal. So when the theme does again, using check_plain($variable) in the template html characters appear as their html escape sequence

example: a logo like "fruits & vegetables" shows up as fruits & vegetables.

Note that the Garland theme contains the same error, but many other themes don't use check_plain() (like zen, fusion, chameleon, bluemarine, pushbutton).