In the United States, it is BAY-ta ("ay" in "day" or "ey" in "they")...In Britain, it is BI-ta or BEE-ta ("e" vowel as in "beet" or "delete").
Other Greek letters with the "e" vowel (e.g. ETA, THETA, etc.) also exhibit these differing pronunciations. As Wikipedia says, "The Commonwealth [British] pronunciation is more naturalised [to English] than the American, which is more in keeping with the ancient Greek." Interesting, modern Greek uses a vowel closer to the British pronunciation, as opposed to ancient Greek or American English.
see:
Wikipedia article "American_and_British_English_differences#Slight_lexical_differences"
