WP_Query: offset ignored when posts_per_page is -1?

posts_per_page => -1 is equivalent to 'nopaging' => true, which basically means no LIMIT clause is used at all.

The workaround is to set posts_per_page to a large number.

This is a limitation of the LIMIT syntax itself:

http://dev.mysql.com/doc/refman/5.0/en/select.html