The version of Pidgin that is in the Lenny repositories has some problems. It’s an older version and does not work with Yahoo messenger, usually you can’t get the proper certifcate from Yahoo in Pidgin. More here
I attempted to install it myself using the version of Pidgin for Sid by downloading the files from Debian. I ran into dependency issue after issue and finally decided just to go with backports. This is the easiest way to install Pidgin 2.6 on Debian Lenny.