My dictionary, The Oxford Complete Wordfinder, has 'till' listed 4 times.* According to this dictionary 'until' is used in formal style and at the beginning of a sentence, otherwise 'till' is perfectly acceptable.
*1. up to or as late as
2. a drawer for money in a store or bank
3. prepare and cultivate
4. stiff clay containing boulders, sand, etc., deposited by melting glaciers and ice sheets
ETA: Just as a point of interest, in the Anglican Book of Common Prayer, the marriage ceremony has this line:
I M. take thee N. to my wedded Wife, to have and to hold from this day forward, for better for worse, for richer for poorer, in sickness and in health, to love and to cherish, till death us do part, according to God's holy ordinance; and thereto I plight thee my troth.
So even in formal style 'till' is used.