Haha ... I remember doing that too!!  I thought my husband was going to die of embarrassment!!  To this day I still don´t know what it means, but I know, it´s baaadddddd ...

 What sign was it ?

Once a positive gesture wishing luck and fertility to the recipient, the Fig -- a fist with the thumb sticking up between the index and middle finger -- is now the equivalent of a bird ( the flip off ) in Turkey.

O i know it.... this is bad in my country too but as i remember from childhood its made sometimes to the children when they expect to see what you are hiding..and in fact you show them this sign..

In Canada, it is a cute childhood game ... you (the adult) pinch the child´s nose and then show them this sign and say ´I got your nose´ ... the half-hidden thumb is supposed to be the ´nose´ you just pinched off of their face. 

Yikes!!! I play this with my twins all the time, they love it, they laugh so much! Think I best knock it on the head when we go to Turkey so , who knew something so innocent could mean something so different!  

http://www.business-with-turkey.com/tourist-guide/turkish_culture_shock.shtml

 About body language...

http://www.business-with-turkey.com/tourist-guide/turkish_body_language.shtml 

Has anyone ever heard the phrase "when in Rome do like the Romans do"?

It does make sense.  After all, however silly or ancient those customs are they are their customs, it is their country.  It will do well for people to learn them prior to any visit for several reasons.  One of them being, Turks will respect and will take you under their wing and protect you from those that wish to take advantage of you.  I´ve been to Turkey several times and in each case due to my consideration I´ve made friends that took care of me.  I even married a turk, and although we had our share of marital issues none of them were related to being a Turk.  Not once have I had a bad experience, they always steered me clear of bad situations, even when another handsome Turk guy tried to take advantage of me, my other Turk friends kicked his a$$ really good, he won´t evven look in my direction.  So trust me it is worth to learn any country/people customs.  Hey I had to put my two cents in. 

thank you for your two cents, dear

yes, heard that saying.

however, I personally dont know any easterner learning the customs of the west when they are about to visit. Its even oppposite, when easterner visits the west he/she insists on continuing to behave like he/she never left home. and everyone else is expected to tolerate, wow even to adjust to the guest´s behaviours.

you see I know a person, an ESOL teacher, she told that once she was given a group of arab men. arabs were furious to see a woman to teach them. they refused this and that. they refused slideshows where there were women. they made fuss about everything.

when I go to another country, I just want to be a guest, want to enjoy sightseeing, food and talk to people.

Thanks for the great links ReyhanL!