b2evolution is now integrated in the "unstable" release of Debian and should soon enter "testing".

To install an up to date version b2evolution on your debian system, update your package list:
apt-get update
then, simply run:
apt-get install b2evolution

For more information or download the package manually, visit http://packages.debian.org/b2evolution

Thanks a lot to Arnaud Guiton who created and maintains this package.