According to wikipedia, season 6 Part 1 should be released somewhere around Summer 2009 and Part 2, Autumn 2009.
I'm not sure if that's 100% accurate as I'm not as familiar with release patterns in different regions but I believe you guys started getting season 6 on television recently, so it would seem to make sense that the DVDs will be out late next year for you. Hopefully this information is helpful but UK/Region 2 fans, if I'm way off, feel free to clarify.