Ubuntu releases...

Here is a short list of the ubuntu releases with version numbers and codenames:

4.10 Warty Warthog (Released in Oct 2004 - The First Ubuntu Release)
5.04 Hoary Hedgehog (Released in April 2005)
5.10 Breezy Badger (Released in Oct 2005)
6.06 Dapper Drake (Released in June 2006 - Long Term Support)
6.10 Edgy Eft (Released in Oct 2006)
7.04 Feisty Fawn (Released in April 2007)
7.10 Gutsy Gibbon (Released in Oct 2007)
8.04 Hardy Heron (Released in April 2008 - Long Term support)
8.10 Intrepid Ibex (Released in Oct 2008)
9.04 Jaunty Jackalope (Released in April 2009)
9.10 Karmic Koala (Planned Release - Oct 2009)

Regular releases are supported for 18 months, LTS releases are supported 3 years on desktop and 5 years on server versions. As you probably found out - release cycle is 6 months.