The UDS-N (Ubuntu Developer Summit - Natty Narwhal) started today and Mark Shuttleworth already announced that Unity will be default for Ubuntu 11.04 desktop:
"11.04 will have Unity as the default desktop for new users" - via popey
I'm not exactly sure what "new users" refers to, but it might mean that those who will upgrade will use the old Gnome interface instead of Unity. Also (important!), Unity will only be default if your graphics card supports it:
It is going to be a primary focus this cycle to enable Unity on as many chipsets as possible.
(Neil Patel, Technical lead for Ubuntu Netbook Edition and Unity, Desktop Experience Team)
Unity is the current Ubuntu Netbook Edition interface and has received a lot of criticism since UNE 10.10 came out such as not being finished, poor performance and the lack of customization.
Also, a new icon theme will be designed especially for Ubuntu. It will not be ready for 11.04 but the work will probably start now and the new icon theme should be ready by Ubuntu 12.04.
Update: and a surprise (I for one definitely didn't see that coming): Unity will use Compiz instead of Mutter!
If you don't know what Unity is, check out our Ubuntu Desktop and Netbook 10.10 post (lots of screenshots and Unity video inside).
For more info, stay tuned!