Hi Krisztian,
             I am a developer working (along with other developers) on the Nokia N900 Open Hardware Manager. We are trying to find a way to enable the jackbias setting for headphones for use in things other than phone calls such as media players.

We have located source code for the various Open Hardware Manager bits that matches the binaries shipped with the N900 (including libprolog, libdres, and ohm-plugins-misc) but we can't find a way to enable jackbias without pretending to be a VOIP call (which has unwanted side-effects)
Some analysis with swi-prolog shows that the jackbias seems to be handled by logic inside the compiled prolog and dresrc files (policy.plc and policy.dresrc) but we cant find any mention of jackbias in the "policy-settings-basic" source code on gitorius.

Is there any information you can share regarding where the jackbias logic is, how it works and how we might be able to turn it on without needing to pretend to be a VOIP call?

This forum thread http://talk.maemo.org/showthread.php?t=83447&page=4 contains our research into this so far but we have not been able to find a solution as of yet.