To recap some of the Alpha 1 changes, there is a new (non-final) theme, a massive package import from Debian, and the latest unstable packages from the GNOME 2.23/2.24 branch. While not completed, a number of new Ubuntu features are being planned such as improved 3G networking, Live USB image creator, and enhanced dmraid "Fake RAID" support.
In Alpha 2, the kernel used is based upon Linux 2.6.26-rc8 and the latest bits for X.Org 7.4 with X Server 1.5 are now in place. X.Org 7.4 has yet to be officially released, but it's getting close. For some of the X.Org 7.4 changes, checkout The Progress Of X.Org 7.4 and X.Org 7.4 Creeps Closer To Release.
The Ubuntu 8.10 Alpha 2 release announcement (with download links) can be read on ubuntu-devel-announce. Intrepid Alpha 2 images are available for Ubuntu, Kubuntu, Edubuntu, and Xubuntu. Like Alpha 1, LiveCD images for Intrepid Ibex aren't yet available, but they should start appearing with Alpha 3. Ubuntu 8.10 Alpha 3 is planned fo release in two weeks on July 24.