Kosher food, as well as food for other dietary needs, is available on most long haul flights and must be pre-ordered. Some pax say the kosher (and other special meals) are of better quality, but that is personal opinion. I really couldnt imagine passengers ordering a Kosher meal if they did not observe the Jewish dietary laws, but who knows?
One thing to think about, if you are flying in economy and have ordered a Kosher meal, you will generally lose any chance that you may have had for being upgraded (ie, overbooking, FF
, etc) since the aircraft will not be properly catered (Kosher meals vary by class).