In the Japanese Campaign, the Geisha houses in addition to the four happy faces mentioned in the civilipedia, also grants a 50 % increase in production.
A proof that happy workers work harder ? :p

Grimjack

1) STart up Japanese campaign
2) Reserach into Geisha houses
3) Verify in Civilopedia that no mention of shields is made
4) Build a Geisha house
5) Watch the new found happiness and shields.

are you sure its not just because fewer entertainers are now needed and they went to work tiles?

I think alpha probably has the right explanation here. I've checked out a Geisha House and I can't see any production increase as such, just the content faces and reduction in war weariness that it is supposed to produce.