I have seen this post What are valid values for "data defined" labeling settings?. Does anyone have any insight to the values within attribute tables required to satify the expanded "Data defined settings' in the labeling dialogue box. I am using 1.9
Sorry @LandArch, but those new data defined settings are not yet documented in an organized manner. More data defined settings will be added before the 2.0 release. When the feature freeze for 2.0 comes down, I will be trying to fully document all of the changes to the new generation labeling engine before the string freeze happens, approximately a month later.
In the meantime, and since things are going to be in flux for at least the next month, your best bet is to read the comments in the QgsPalLayerSettings declaration and parse the logic in its definition, then contrast that with how the GUI for those settings is designed and the other referenced answer.