It is of course possible to add 'wheezy' ( and / or 'sid' ) binary to
/etc/apt/sources.list , but it will trash Debian Lenny.
( 'drush' would then install 100 wheezy packages as dependency.)
But this will work smoothly :
1) Add this line to /etc/apt/sources.list :
deb-src
ftp://ftp.funet.fi/pub/linux/mirrors/debian/ sid main non-free contrib
( Which is for the source code only.)
2)
# apt-get build-dep drush
( And as you probably have all the dependencies, nothing gets installed.)
# apt-get -b source drush=4.4-1
A Debian Lenny package is created in the current directory : drush_4.4-1_all.deb
( All this only takes a few seconds.)
3) (Two dependencies : # apt-get install php5-cli php5-common )
# dpkg -i drush-4.4 drush_4.4-1_all.deb
..