The etch-and-a-half release is essentially just the most recent point release for etch, but with a more recent kernel (version 2.6.24). The main purpose of this release is to allow installation of etch on systems not supported by the 2.6.18 kernel that is used in regular etch installs.
Read more about it here
For new installations of etch-and-a-half the current release of Debian Installer for lenny is used, with minor changes for compatibility with etch.
Please read the notes below carefully before installing etch-and-a-half.
* Because the current lenny installer is a beta release, it may have some errata you would not expect in a stable release. See the errata section at the bottom of this page for details.
* The lenny installer includes a number of changes from the etch installer. See the release announcements for the beta1 and beta2 releases of the installer for an overview of changes.
* This includes changes that affect preseeding (automated installations). See the appendix on preseeding in the Installation Guide for lenny (linked below) for correct preseeding options.