In PostGIS 2.0 function SetSRID is removed (it was deprecated in 1.2.3).
Mapnik 2.0 seems to still use it though (updated today from http://ppa.launchpad.net/mapnik/nightly-trunk/ubuntu)
I see that it is used in file postgis_datasource.cpp
Corresponding ticket in Postgis:
postgis: remove last usage of deprecated functions - fully supporting…
… postgis 2.0 - closes #1083
thanks, nice catch! (Somehow I missed that during testing)