"He has yet to" vs. "he is yet to"
- He has yet to receive an appointment.
- He is yet to receive an appointment.
Is there any difference in meaning? Is one more correct than the other?
The string has yet to is 7 to 8 times more frequent than is yet to in both the Corpus of Contemporary American English and the British National Corpus. The preference is clear, but both are grammatical and there is no difference in meaning.
"He has yet to receive an appointment" states that the person being referred to has not, at this point in time, "received an appointment", but is expected to.
"He is yet to receive an appointment" does not, at least to me, make grammatical sense. It would possibly make more grammatical sense to say "He is yet to be a person who has received an appointment", although Googling the phrase "he is yet" with quotes yields very little evidence (besides a bible verse) that the phrase is used regularly.
As StoneyB noted in his comment, this answer is simply based on what sounds right to me, rather than what may make any grammatical or syntactic sense.
When referring to something that will happen, but has not happened yet, I would use the phrase "has yet". This is your first example, and it implies that you are waiting for such a thing to happen.
That is not to say that the second example isn't used, but simply would be in a different context. Let us use the example sentence, "He is to disable the alarm." This sentence would most likely be used in a context of some sort of briefing, describing planned events in the future. (Also, note that this does not make use of the word "yet". This sounds wrong to me and I expect it may not be grammatical.)