If libfaad0 - 2.7.0 is installed then you can't install the 2.6.1 libfaad-dev until it's removed (synaptic and or apt-get won't downgrade off of an attempted install of a -dev

So either remove/downgrade the 2.7.0 libfaad0 first and then install or ...

just go ahead an install the libfaad-dev (2.7.0... ) from your ppa, ffmpeg isn't going to care whether you have the 2.6.1 or 2.7.0 version, all that matters is the lib and -dev are the same version.

( you don't need to re-add the ppa if you decide to go with the 2.7.0 version, just go here and direct download and install the libfaad-dev, expand the arrow for faad0.....karmic3