The four-letter name of the Israelite Deity, called the "Tetragrammaton," was, to all appearances, once pronounced freely. Over the centuries, however, it has become shielded by many degrees of what scholars creatively call "Tetragrammaton avoidance." When the Bible was translate into Greek around the beginning of the common era, the translators substituted the word κυριοs, "Lord," for the Divine Name. By the time the biblical text was fully vocalized, the Tetragrammaton had been replaced by the Hebrew word for "Lord," adonay, in liturgical readings. To indicate the correct reading of the Divine name, the Tetragrammaton was written with the vowels of adonay (yielding the erroneous transliteration "Jehovah"). More recently, Jews began to avoid even adonay in non-liturgical contexts, substituting euphemisms such as hashem ("the Name"). The Tetragrammaton is also avoided in writing. An early substitute consisted of two yods (the first and third letters of the Tetragrammaton). That, however, was too close to the original for comfort, so today, the letter hay, representing "Hashem," is a more prevalent choice. The very pious will avoid even hay, since it is one of the letters of the Tetragrammaton, preferring dalet, the letter preceding hay in the Hebrew alphabet. Sometimes, even non-Hebrew names of God are regarded as too sacred to be written. Many Jews substitute "G-d" for "God;" DH has even seen "Hash-m."
At this point, no one really knows how the Tetragrammaton was originally pronounced, although scholars have their (highly speculative) theories. There is, however, a conventional pronunciation used in academic circles, based on what one might call an educated guess. This places scholars with traditional Jewish leanings in an awkward position. There are times when using a proper name for the Deity is warranted, and departing from the convention to use circumlocutions or alternative euphemisms can be extremely distracting. One Jewish scholar of my acquaintance pronounces the Tetragrammaton on the grounds that he is certain that the conventional pronunciation is incorrect. Another occasionally uses "Hashem" at the risk of sounding unscholarly; a third is reputed to have said, "I'll just call him Jimmy."
I have yet to come up with a personal solution, and this has, on occasion, resulted in considerable awkwardness. Once, I was asked about a book with the Tetragrammaton in the title, and I stood there, dumbly, as though I couldn't remember it. Recently, I became so frustrated at my inability to communicate that I abandoned my principles and pronounced the Divine Name. Later, I reassured myself, noting that I hadn't articulated the medial hay, and in any case, I didn't see how the final vowel could possibly be a long /e/, and even if was, it would have been pronounced as short /i/ when the Name was actually used, etc.
All of this, of course, entirely misses the point. "Tetragrammaton avoidance" is supposed to be about regarding the Deity with a certain degree of reverence -- something that biblical scholars and aspiring scholars rarely do. Most of us are religious in some sense, but at some point, tearing the Bible to shreds and attempting to reconstruct the vowels of the Tetragrammaton does take its toll. The sense of mysterium tremendum so essential to religion inevitably begins to dissipate. Then, when we need it -- say, on the Jewish Days of Awe -- it is ever so difficult to recapture.
* DH thinks that this would be a good name for a rock band.