The Daily Mail took the duchess formerly known as Kate Middleton to task in an article yesterday for writing 'till. The article assumed that she had confused it with 'til, short for until.
There are nearly as many instances of 'till on the British National Corpus as there are for 'til, so Kate is in good company. Moreover, at its entry for till, the preposition (without an apostrophe), which is a synonym of until, the OED includes the spelling 'till among the ten or so different spellings of the word that have been noted over the centuries.
The spelling untill, instead of until, was in common use until the 17th century -- all three of the Shakespeare citations at the Until entry in the OED are spelt with two Ls.
Regarding the Daily Mail article, the phrase 'the pot calling the kettle black' comes to mind. The Mail's spelling and grammar are often dreadful, and, as this Anorak article reminds us, the newspaper has made many more serious errors itself in the past.