I agree with cheerio that telling it early is better than late, and being late is better than never. On the other hand, I can also understand icicle's unsettling fear in breaking the news to her fiance now when the relationship is moving to the next phase of marriage. If I were to find out that my bf has been married before and yet has deliberately kept me in the dark for a long time, I wouldn't be happy about it. It's not so much of not being able to understand and accept his past but I would question his character and intention.
I have been upfront about my divorce right from the start. Family, relatives, friends, colleagues, neighbours and bf all know that I am divorced though most did not know the details at length. Divorce is a reality for me and I didn't see why I should hide it from anyone. In fact, to become more resolute in my decision and to move on as soon as possible, I chose disclosure.