"Till death do us part"

Every time I see this expression, I can't help thinking it's grammatically wrong.

Till death do us part

Is it grammatically acceptable? Why is it used extensively in this form?

The phrase is quite old: it was part of the Book of Common Prayer from 1662 (see http://www.eskimo.com/~lhowell/bcp1662/occasion/marriage.html). (For all I know, it could be older even than that.) But fixing it as the official language of a ceremony cemented the phrase in that form, even as the language changed around it. It's probably best viewed as idiomatic; you wouldn't want to say something else using the same form without a very good reason, but that particular phrase is a widely recognized feature of the language.

(Note, by the way, that the form in the BCP really was with "till" rather than "until"; this isn't surprising, since "till" is actually the older of the two words.)

It's the present subjunctive. In older forms of English, most conjunctions took the subjunctive; thus we would see, "till the Son of God appear", "before I be put to death", "If it be the last thing I shall ever do", and so on.

All of the answers omit a key point, or at best imply it. While "death" here is the subject, most people take "death" to be a temporal indication. This is why they expect the pronoun to be "we": I promise to stay with you till (the time of) death. Of course, it really means "until death separates us."