Since about a week the Debian project invites everyone to join in the countdown to the brand
rustynew version of Debian.
While waiting for the official squeeze release, one can already see the changes to the distribution by looking at the official "What's new" page that's offered by Debian.
While some architectures were dropped, HP PA-RISC ('hppa'), Alpha ('alpha') and ARM ('arm'), the Debian team added a GNU/kFreeBSD release for x86 and x86_64 as a technology preview.
We already installed our Debian squeeze test system some months ago, without finding big pitfalls.
Have fun updating your systems over the next months :)