In an Italian interview Stef refers to "the singer of Giant Sand" as one of the members of the Song With Other Strangers project. Probably he means John Parish, who produced some Giant Sand records... which would make sence because Parish is listed as one of the musicians participating in this project.

Another member of this early-2005-project was Manuel Agnelli, lead singer of Afterhours (official, myspace, wiki) and also a friend of Stef. On the upcoming Afterhours-record ("I Milanesi ammazzano il sabato", to be released May 2nd) Stef's doing backing vocals on 4 songs.

Songs with other strangers was:
Manuel Agnelli (Afterhours), Marta Collica (Sepiatone), Cesare Basile, Hugo Race (Nick Cave and the bad seeds, True Spirit), John Parish (who worked with PJ Harvey, Eels, Sparklehorse, Giant Sand, Tracy Chapman) and Stef Kamil Carlens (once dEUS, now Zita Swoon)

some pictures: here

+ Trivia: 
1/ On the previous official Zita Swoon-site, you could download two songs by Cesare Basile ('L'alberto di giuda' and 'In coda').
2/ There were plans to release a cd with SWOS-recordings (april 2006)... but I never found one, did you?
3/ My previous personal blog is topping the SWOS-google search (I copied the article from Mescal.it (italian label))

+
Neli wrote an article about the project (with tour dates). It's in Japanese, but google translates (some of) it for you.