Assume Apple (AAPL) today is worth $391 billion (nearly 40% more than its current market cap), as the analysts at Trefis estimate it to be.

Assume as well, as they do, that the iPhone represents 53.5% of that value.

Then it follows, according to them, that the iPhone, were Steve Jobs to spin it off as a separate company, would be worth $209 billion.

How does that compare with the largest companies in, say, the automotive industry, or pharmaceuticals, or media?

It would dwarf them all. In fact, team Trefis could find only ten companies on the planet — three of them Chinese — that are more valuable than the iPhone. They are

• AT&T (T): worth $224 billion in Trefis’ valuation
• Petro China (energy): $225 billion
• China Construction Bank (banking): $229 billion
• IBM (IBM): $234 billion
• BHP (mining): $236 billion
• Procter & Gamble (PG): $243 billion
• Microsoft (MSFT): $262 billion
• ICBC (banking) : $265 billion
• Exxon Mobil (XOM): $349 billion
• and, of course, Apple itself: $391 billion

[Follow Philip Elmer-DeWitt on Twitter @philiped]