Ubuntu's next release is 7.10 aka Gutsy Gibbon, which is due in October of this year (the 7 comes from 2007 and 10 comes from October). Before 7.10 can come out, 8.04 code named Hardy Heron has been announced. This will be a Long Term Support (LTS) release. From Jono Bacon's blog:

"I am delighted to have the pleasure of announcing the Hardy Heron (Ubuntu 8.04), the next version of Ubuntu that will succeed Gutsy Gibbon (Ubuntu 7.10, due for release in October 2007). Not only will the Ubuntu community continue to do what it does best, produce an easy-to-use, reliable, free software platform, but this release will proudly wear the badge of Long Term Support (LTS) and be supported with security updates for five years on the server and three years on the desktop. We look forward to releasing the Hardy Heron in April 2008."

Read more at http://www.jonobacon.org/?p=1017