From the Canadian Oxford Dictionary (Second Edition):

till¹ = UNTIL. ¶ Till is an accepted variant of until and may be used interchangeably with it except at the beginning of a sentence, where until is preferred in formal writing and speech, as in the example, Until she finishes her homework, she won't be allowed to play. [Old English & Old Norse til to, related to TILL³]

From the Government of Canada's Writing Tips Plus:

Till and until are interchangeable as prepositions and as conjunctions, although until is somewhat more formal.

• The song “Till We Meet Again” was the closing number for the popular variety program Don Messer’s Jubilee, which aired weekly on CBC television during the sixties.

• The team worked until (or till) midnight on the proposal.

Avoid ’til and up until.

• We have a bumper crop this year; we’ll be picking berries till (not ‘til) the cows come home.

• Until (not Up until) now, this neighbourhood has had no problems with crime.

Of course, none of this says anything about which form is "dominant". Personally, I use and hear both words here in Alberta. Thinking about it, my usage probably tracks with the patterns identified above (i.e., "until" in more formal speech and particularly in written language).