Because of icq protocol change it is necessary
to backport kopete on sarge to use it.

0) first add in in /etc/apt/sources.list
 deb ftp://markus-raab.org/apps ./

1) now kill kopete

2) deinstall
 aptitude purge kopete

3) backup user configuration (PER USER!)
 cd ~/.kde/share/config
 mkdir kopete && cp kopete* kopete
 cd ~/.kde/share/apps
 cp -r kopete kopete~
 cp -r kopete_cryptography kopete_cryptography~
 cp -r kopete_history kopete_history~
 cp -r kopeterichtexteditpart kopeterichtexteditpart~
 
4) set apt-pinning (/etc/apt/pinning)
 Package: kopete
 Pin: release a=build
 Pin-Priority: 999

5) Now install kopete 0.12.0
 aptitude install kopete

Have fun chatting!