Fedora infrastructure depends on python-bugzilla in various ways:
http://lists.fedorahosted.org/pipermail/python-bugzilla/2012-June/000001.html

Red Hat bugzilla originally had a totally custom API. Much of that is
being dropped in 2013, API conversions outlined here:
https://bugzilla.redhat.com/show_bug.cgi?id=822007

Current RH bugzilla docs:
https://bugzilla.redhat.com/docs/en/html/api/

Externally facing RH bugzilla instance that doesn't send email and is
refreshed periodically. This is what is used in the functional test suite:
http://partner-bugzilla.redhat.com

Some trackers in the wild to use for API testing:
    bugzilla.redhat.com
    bugzilla.mozilla.org
    bugzilla.kernel.org
    bugzilla.gnome.org
    bugs.freedesktop.org
    bugzilla.novell.com
    bugzilla.zimbra.com
    bugzilla.samba.org


  Upstream timeline
  =================

Here's a timeline of the evolution of the upstream bugzilla XMLRPC API:

Bugzilla 2.*:
    No XMLRPC API that I can tell

Bugzilla 3.0:
    http://www.bugzilla.org/docs/3.0/html/api/index.html
    Bug:
        legal_values
        get_bugs:
            returns: id, alias, summary, creation_time, last_change_time
        create
    Bugzilla:
        version
        timezone
    Product:
        get_selectable_products
        get_enterable_products
        get_accessible_products
        get_products
    User:
        login
        logout
        offer_account_by_email
        create

Bugzilla 3.2:
    http://www.bugzilla.org/docs/3.2/en/html/api/
    Bug:
        RENAME: get_bugs->get, get_bugs should still work
        add_comment
    Bugzilla:
        extensions
    Product:
        RENAME: get_products->get, get_products should still work

Bugzilla 3.4:
    http://www.bugzilla.org/docs/3.4/en/html/api/
    Bug:
        comments
        history
        search
        update_see_also
    Bugzilla:
        time
        DEPRECATED: timezon,e use time instead
    User:
        get
    Util:
        filter_fields
        validate

Bugzilla 3.6:
    http://www.bugzilla.org/docs/3.6/en/html/api/
    Bug:
        attachments
        fields
        DEPRECATED: legal_values
    Bugzilla:
        timezone now always returns UTC+0000

Bugzilla 4.0:
    http://www.bugzilla.org/docs/4.0/en/html/api/
    Bug:
        add_attachment
        update
    Util:
        filter_wants

Bugzilla 4.2:
    http://www.bugzilla.org/docs/4.2/en/html/api/
    Group:
        create
    Product:
        create

Bugzilla 4.4:
    http://www.bugzilla.org/docs/4.4/en/html/api/
    Bug:
        update_tags
    Bugzilla:
        parameters
        last_audit_time
    Classification:
        get
    Group:
        update
    Product:
        update
    User:
        update
    Util:
        translate
        params_to_objects