So my friend got a L permit last November, and is obviously due to expire at the end of Nov 2010. He is no longer living in Switzerland but still has the permit (he never de-registered). Does he need to come back to Switzerland to de-register or can he just send his permit back in the post or does he need to do anything at all seeing as he only has a 1 year L permit?
If someone doesn't provide some conclusive information here, I'd try and de-register from the Basel-Stadt office in town. When I was here for a year under an L-permit back in 2001 I just left (or actually forgot) and tried to talk to the police at Geneva airport to which they weren't interested. Anyway, I've been back for 7 years and not had any issue--although not lived in Basel-Land since (was on a B-permit in BS and now on a C in ZH).
Don't forget to cancel other items such as television licence, etc. I've heard some nasty stories.