It apparently was dat-uh in the U.S. for a very long time, basically until TNG. In fact, the first time Patrick Stewart pronounced it the way we know it now, Brent Spiner asked the producers if it was pronounced the other way and he and Stewart debated it and the response from on high was, “from now on, whoever says the name first: that’s how it’s pronounced.”
It apparently was dat-uh in the U.S. for a very long time, basically until TNG. In fact, the first time Patrick Stewart pronounced it the way we know it now, Brent Spiner asked the producers if it was pronounced the other way and he and Stewart debated it and the response from on high was, “from now on, whoever says the name first: that’s how it’s pronounced.”
Edit: Here’s him telling the story- https://www.youtube.com/watch?v=xeqTMTOxid8 (I got it slightly wrong, it came from Gene himself.)