This bug report documents the problem well:
The package pidgin-data has only 1 installation candidate from backports now: 2.6.x. The 2.5.x version has gone.
But, there is only a 2.5.x version of pidgin available for x86, which requires a 2.5.x version of pidgin-data.
So currently a working pidgin is not currently installable! (one that works with yahoo servers).
What is the holdup on 2.6.x versions of pidgin for x86 (and platforms other than powerpc) ? And how did pidgin-data's older version get dropped prematurely?