The release candidate installer for Debian 6.0 Squeeze includes new artwork, the latest Linux 2.6.32 kernel is used, support for new computer systems (two QNAP ARM devices and a PowerPC-based YDL PowerStation), and support fixes for other systems and platforms.
Compared to Debian 5.0 Lenny, the installer for Debian 6.0 supports taking care of the installation of recommended Debian packages, Btrfs and EXT4 file-system support, easier partition configuration, and more.
With Squeeze there is also the interesting Debian GNU/kFreeBSD distribution that marries the FreeBSD kernel with a Debian user-land. The Debian installer for this port is also capable of installing to a ZFS file-system, but alas that's not an option for Debian GNU/Linux.
With Debian 6.0 they have also taken a tougher approach against closed firmware.
The Debian Squeeze Installer Release Candidate message can be found at Debian.org.