Even Ubuntu 10.10 “Maverick Meerkat” has not been released yet, but the release schedule of Natty Narwhal is already out. According to it, Ubuntu 11.04 is scheduled for release on 28th April 2011.
This time around there is a bit of a change in the pre-final releases. With Ubuntu 11.04, there will be five alpha releases instead of the three during the Maverick development cycle. The number of beta and RC remains same at one each.
Anyway, here is the release schedule:
|Alpha 1||4th Novembe 2010|
|Alpha 2||2nd December 2010|
|Alpha 3||6th January 2011|
|Alpha 4||3rd February 2011|
|Alpha 5||3rd March 2011|
|Beta||31st March 2011|
|Release Candidate||21st April 2011|
|Final Release||28th April 2011|
You can see the detailed schedule at the Ubuntu Wiki.