Ubuntu team is already planning for 10.04 "Lucid Lynx" LTS, it will be released in April 2010. Here is a list of dates when the Alpha and Beta versions will be released.
Ubuntu 10.04 "Lucid Lynx" LTS will be released on the 29th of April 2010.
Release Dates:
December 3rd, 2009 – Alpha 1 release
January 7th, 2010 – Alpha 2 release
February 4th, 2010 – Alpha 3 release
March 4th, 2010 – Beta1 release
April 1st, 2010 – Beta2 release
April 15th, 2010 – Release Candidate
April 29th, 2010 – Final release of Ubuntu 10.04 LTS
Complete LucidReleaseSchedule: Here.