Ah ok!
The latest released version is 0.12.2, so i guess for kopete 0.12.3 you need to grab the source code from KDE's SVN.
I think you could contact the kopete developers if this is true, and how you can proceed downloading and compiling the SVN source.
kde 3.5.6 will be out next week, dunno if kopete will be upgraded to 0.12.3 in that release.