this may be rubbish, but...
i looked at the table relationships of the wordpress tables on this link Database_Description
i can't confirm the relationship between wp_posts and wp_term_relationships, so for now i have assumed object_id on the relationships table is ID on the posts table.
based on this massive guess of an assumption, i have collated a sql statement with a few inner joins, which at present should return all the rows in the posts table where the slug is 'site-news-item'
if my assumption isn't total rubbish, you may be able to use this to get what you want.
SELECT * FROM wp_posts INNER JOIN wp_term_relationships ON wp_term_relationships.object_id = wp_posts.ID INNER JOIN wp_term_taxonomy ON wp_term_taxonomy.term_taxonomy_id = wp_term_relationships.term_taxonomy_id INNER JOIN wp_terms ON wp_terms.term_id = wp_term_taxonomy.term_id WHERE wp_terms.slug = 'site-news-item'
failing that, i either need to play with wordpress a bit, (long overdue) or you could send me a pm with your sql dump and i will have another try.
as for now, it's Sunday night and i am going to grab a glass! :-)
