What is the significance of the title "The painted veil"?

The movie is based on the 1925 novel of the British author W. Somerset Maugham. The title is taken from Percy Bysshe Shelley's sonnet which begins "Lift not the painted veil which those who live / Call Life".

This entry on answers.com offers an explanation

At a wedding, a veil is usually white, representing the innocence of the person wearing it. White is pure and has not yet been violated. In the novel the veil is painted, which means that it is not an innocent veil the characters are wearing. Maugham used this concept to show how all of society and the people living in it are tainted. The only characters' in the novel that wear white, literally speaking, are the nuns. In the novel one can read how they are all dressed completely in white, except for a red cross on their chests. It is not completely clear if the nuns are in fact wearing painted veils like the rest of the characters. Nuns are supposed to be innocent and saint-like, but maybe Maugham wants to say that just because they are working for God does not mean that they are innocent. Indeed Waddington questions their true nature.

He says that he tried to make them leave, for their own safety, but they would not, because they all want to die as martyrs (Maugham 101). Another thing that can make one question the nuns' motives would be their imposing Catholicism on the townspeople and the orphans. Do they only help and take care of the children as long as they can spread their religion? However, another interpretation is also possible. Kitty believes that they are all saints, and they are the reason why she decides to make a change in her life and give some back. The nuns do help Kitty lift her own veil. They are the reason why she sees the truth about her own shallow and selfish lifestyle.