Most people I have asked say that as long as they believe (with valid reason, ie govt certification) that their meat is Halal, then essentially it is, but I did meet one very very conservative guy who thought that if he ate non-Halal meat, even if he genuinely didnt know it was non-Halal, it would be a sin serious enough to send him to hell.
He also believed absolutely that Halal certified meat that came from a non-muslim country (ie had been handled by non-muslims) would not meet his Halal requirements. As a result, he will eat nothing that he cannot verify - KFC is OK, because their chickens come from Al Ain and were therefore killed by and under the supervision of Muslims, but McDonalds is out because the beef is from Argentina or Brazil.
However, I know that he (at least in the UAE) is one of a very small minority. He seems to be one of the most sensitive, understanding and genuinely good people I have ever met, and I have tremendous respect for the effort he puts into living his life in the best way he can, but still find it infuriating that he considers that touching my hand would make him unclean. Makes me want to Tango him.
Hows that for going off-topic....