United Kingdom
Season 1-7 are available on DVD in the UK and Ireland. Part 1 of season 8 is available with the second part of season 8 coming out on February 16th of 2009.
United States
Season 1-8 are availble on DVD in the US and Canada.
Australia
Season 1-7 are available on DVD now.
Blu-Ray Disc Releases
Season One will be released on May 12, 2009 on the high definition format, most probably in the US first.