I was trying WordPress with 'magic_quotes_runtime' enabled, and it cause an error like this when I enter admin page :
Warning: Invalid argument supplied for foreach() in ...../wp-includes/capabilities.php on line 19
I traced the error and found that variable '$this->roles' is not an array, but it is a string with many backslash in it. It's look like that WordPress doesn't detect 'magic_quotes_runtime' setting in php.ini, so I try to disable the 'magic_quotes_runtime' in php.ini and it works.. ;)
Another way to disable 'magic_quotes_runtime' is by adding :
ini_set( 'magic_quotes_runtime', 0 );
in 'wp-config.php' file.