It HAS happened before. However, there have been many things that have changed my mind. This represents a major leap. Everyone knows how much Mac folk dislike Microsoft and look for excuses not to use their software - Opera may be that excuse. Almost all of the major sites on the net have been cleaning up their woeful html the last six months.
Opera is now free and they can test their pages in it.
It is now rare for me to write a website and ask them to clean up their html. A year ago, I did it 10-30 times a day. The Opera user base grows by 25k per day and is now over 2 million strong. That put's it's installed platforms over that of WebTv and makes Opera a lock on the 3rd most often used browser. They are tired of MSN competing with them.