Nested meta_query with multiple relation keys
The question was for WordPress 3.0, but just in case someone has the same question for a more recent version, from WordPress Codex: “Starting with version 4.1, meta_query clauses can be nested in order to construct complex queries.” https://developer.wordpress.org/reference/classes/wp_query/#custom-field-post-meta-parameters So, that query should work on the current WordPress version.