The Linux 2.6.28 kernel was released back in December, but the 2.6.29 kernel is introducing mainline support for Btrfs, Intel kernel mode-setting support, and various other new features. Numerous Linux drivers have also been either added or updated within the Linux 2.6,29 tree.
Canonical's Tim Gardner had said on the kernel team's list, "Jaunty will absolutely, positively, and without a doubt, release with a
2.6.28 kernel." Many Ubuntu users -- including us -- would have liked to see the Linux 2.6.29 kernel inside Ubuntu 9.04, but that simply will not happen. Unless you are interested in building your own kernel, users will simply need to wait until Ubuntu 9.10 when it will ship with the Linux 2.6.30 or 2.6.31 kernels.
With Ubuntu 8.10 there was a similar kernel debate about using the 2.6.26 or 2.6.27 kernel, but in that case they ended up choosing the newer version.