The decision has not yet been made but it seems that the release date will be postponed by 6 weeks. Originally Ubuntu 6.04 should be released on 20th April, but Shuttleworth wants to have more time for testing and improvements. The reason therefore is, that Dapper Drake will have extended support and it will be positioned as a enterprise distribution. That means it needs to be suitable for enterprise environments as well.
It is not yet sure if it will keep the name Ubuntu 6.04 as it will be released in June. Maybe we're talking about Ubuntu 6.06 :)