Now that we've started storing stuff in .cache, there's no reason (other than time) not to implement <http://xmpp.org/extensions/xep-0237.html>. It's implemented by the Prosody server, and shouldn't be too hard. It's probably a matter of adding wocky_roster_set_cache_file(), which gabble would call with "${XDG_CACHE_DIR}/telepathy-gabble/${account}/roster.xml", and then just saving a <query xmlns="jabber:iq:roster" ver="">...</query> node in that file.
Ejabberd 2.1.0 implements it as well: http://www.ejabberd.im/ejabberd-2.1.0
*** This bug has been marked as a duplicate of bug 23367 ***
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