I have been looking for this but wasn't able to find anything conclusive using the search function. How is the lowpass frequency determined in the alt-presets (ABR/CBR) ? Is there a look-up table or is it some sort of function of the specified bitrate?

It is a table.

In the current CVS, you'll find it in presets.c in libmp3lame.

In the current cvs, you will find it in lame.c:
http://cvs.sourceforge.net/cgi-bin/viewcvs....viewcvs-markup

The function is "optimum_bandwidth"

Thank you!