Have you seen the recent FR post?
"The top five publicly-traded companies, based on full market values, are:"
1. Exxon Mobil (XOM) - $310.45B
2. Apple (AAPL) - $252.66B
3. Microsoft (MSFT) - $219.19B
4. Berkshire-Hathaway (BRKA) - 206.70B
5. Wal-Mart (WMT) - $193.28B
Passing Wal-Mart happened a while ago. Apple hit $200B back in March. $100B was only three years ago. It's only a matter of keeping up the rate, and even a much slower rate would still do it within a couple years. Whether Apple can sustain that kind of growth is the question.
Apple's revenue and margins have not sustained that kind of growth - classic sign it's a bubble. It may inflate a lot more, but it shows all the signs of a bubble, and the pop will be loud and harsh...
Even you admitted you see know sane reason why Apple should be higher valued than XOM, that it should be the most valuable company in the world. If it closes in on that point, it's bubble all the way!
Lower revenue, smaller profit margins, and declining market share relative to its competitors - those aren't the signs of a company poised to take over the world!
Good time to but their stock then eh.