Need to update some WP Core Trac tickets with refreshed patches since some work has happened between my patches and the attention to the ticket by core WP devs. Will do that tonight, but everything looks on trac for implementation in 3.1 of all four core WP tickets.

Taxonomy Filters: http://core.trac.wordpress.org/ticket/15020
Comments Filters: http://core.trac.wordpress.org/ticket/15019
Meta SQL Filters: http://core.trac.wordpress.org/ticket/15018
Meta Function (get / add / update / delete) Filters: http://core.trac.wordpress.org/ticket/14766