SGU, which shares the day/time slot with Warehouse 13, has to finish its back half of episodes and it's slated to return in April. Once SGU's done, it will be late June and there's almost always a week or two delay between one show's season ending and another's beginning on SyFy. Also, both of W13's seasons have started in July. So, early to mid July.
Executive Producer Jack Kenny stated through his Twitter feed that he is hopeful Warehouse 13 will return for a third season, but no announcement has yet been made.
Season 13 starts January 3, 2012!!
June 13 2010
Warehouse 13 - 2009 The Greatest Gift 3-13 is rated/received certificates of: Netherlands:16
Syfy has ordered 13 episodes for season two, which is set to premiere next summer.
Warehouse 13 - 2009 is rated/received certificates of: Australia:M Netherlands:12 (season 2) Netherlands:16 (seasons 1, 3 and 4) South Korea:15 USA:TV-14 (TV rating)
Warehouse 13 - 2009 Love Sick 3-3 is rated/received certificates of: Netherlands:16
Warehouse 13 - 2009 Stand 3-12 is rated/received certificates of: Netherlands:16
Warehouse 13 - 2009 Insatiable 3-10 is rated/received certificates of: Netherlands:16
Warehouse 13 - 2009 Trials 3-2 is rated/received certificates of: Netherlands:16
Warehouse 13 - 2009 3--- 2--- 1 3-5 is rated/received certificates of: Netherlands:16
Season 3 Episode 12 (or just Episode 37) - "The Flame Alchemist, the Bachelor Lieutenant, and the Mystery of Warehouse 13.