Using Lenny-Backports you can install Pidgin 2.6.2 on Debian Lenny.

Backports are recompiled packages from testing (mostly) and unstable (in a few cases only, e.g. security updates), so they will run without new libraries (wherever it is possible) on a stable Debian distribution. I recommend you to pick out single backports which fits your needs, and not to use all backports available here.

-from backports.org

To add backports to your apt sources do the following steps.

open up /etc/apt/sources.list in your editor of choice (as root)

add the line:  deb http://www.backports.org/debian lenny-backports main contrib non-free

save the sources.list

apt-get update

Now you need to add key from backports.

apt-get install debian-backports-keyring

or

gpg –keyserver hkp://subkeys.pgp.net –recv-keys 16BA136C
gpg –export 16BA136C | apt-key add -

or

wget -O – http://backports.org/debian/archive.key | apt-key add -

Now I would suggest removing the current pidgin install.

apt-get remove pidgin

Once it is removed use the -t lenny-backports flag to install pidgin.

apt-get install -t lenny-backports pidgin

http://www.debiantoday.com/install-pidgin-2-6-2-on-lenny/