OK, so... long story short, it looks like there's a new package (see the end notes for details) named gnome-themes-ubuntu, which contains three of the most popular, community created GNOME themes: Dust, Dust Sand and New Wave. However, none of the new themes is set as default... yet! Without further introduction, here are the new themes...
Ubuntu 9.04 Daily Build 03172009 with the New Wave theme
Ubuntu 9.04 Daily Build 03172009 with the Dust theme
New Wave is created by Anton Kerezov, Dust and Dust Sand are created by Rico Sta. Cruz and Kido Mariano. And, now that we have a new login screen, new notifications and three new nice themes... all we need is a breathtaking wallpaper.
Ubuntu 9.04 Daily Build 03172009 with the Dust Sand theme
Among other new features in Ubuntu 9.04 is the famous GNOME 2.26 desktop environment, which brings lots and lots of improvements in many areas, such as Evolution, GNOME Control Center, GNOME Media, GNOME Power Manager, etc. Ubuntu 9.04's kernel will be based on the latest version of Linux kernel 2.6.28, which will offer support for many new devices (wireless, webcams, etc). Moreover, applications such as OpenOffice.org 3.0.1, The GIMP 2.6.6, Mozilla Firefox 3.0.7, Mozilla Thunderbird 126.96.36.199, Transmission 1.51 or Pidgin 2.5.5 will also be present in the final version of Ubuntu 9.04 (Jaunty Jackalope), due for release on April 23rd.
Don't forget to visit our website next Friday for the Ubuntu 9.04 Beta Screenshot Tour, where we will unveil more of Jaunty's new features!
Later edit: We discovered that the themes will be present in Ubuntu 9.04 and they are all in an official package, named "gnome-themes-ubuntu". Please see the screenshot below for more details.
Ubuntu 9.04 Daily Build 03172009 - The gnome-themes-ubuntu package in Synaptic