We're tentatively scheduling taking MediaWiki 1.5 beta and upgrading the Wikimedia sites for next weekend, circa June 24/25/26.
This will mean some read-only time while the databases are restructured. Exactly how long it will take we're not sure; if possible this will be a 'rolling blackout' with each wiki taken off for conversion in turn while the others remain editable.
-- brion vibber (brion @ pobox.com)