While Ubuntu 9.04 will hopefully be shipping with the Linux 2.6.29 kernel, the latest release candidate for Linux 2.6.28 is currently the default kernel in the Jaunty repository, which is a step-up compared to Linux 2.6.27 in the Intrepid Ibex release.
Perhaps more interesting to end-users than a new kernel is a Firefox upgrade. Firefox 3.1 will be the web-browser in this next Ubuntu release. The Ubuntu package in the development repository is currently based off of Firefox 3.1 Beta 2. Some of the features for Firefox 3.1 and the Gecko 1.9.1 engine that powers it can be found on the Mozilla Wiki.
Ubuntu developers are also transitioning from Mono 1.9 to Mono 2.0 with many of the updated packages already appearing in the Jaunty repository.
Exciting us this week from the changes in the Ubuntu 9.04 package set include the libdrm (the Direct Rendering Manager library) being bumped to version 2.4.1 and the Intel 2.5 graphics driver being introduced. With those updates plus the Linux 2.6.28 kernel, it would be possible to easily build the Intel driver with the Graphics Execution Manager enabled.
Many other packages have also been updated in the past few days -- including NetworkManager and a horde of GNOME software. To see all of the changes, look at the jaunty-changes archives. If you are really interested, there is the jaunty-changes mailing list, which is very high traffic as you'll be notified anytime a new package is accepted for into 9.04.