Yen Press has just announced that they have picked up the license for volume 6 of Yotsuba&! Volume 6 will be released in September 2009. Check back for more details later.
ETA: To be clear, Yen has picked up the license for the series going forward and will be releasing subsequent volumes every few months. I believe the series is up to vol. 9 in Japan. Yen Press honcho Kurt Hassler said that they would be looking to pick up the license for the older volumes as well.