Microsoft's market cap (the value of all shares of stock combined) was $219.2 Billion as of market close this afternoon.  Apple, whose stock has been climbing rapidly, hit a market cap of $222.1 Billion.   This makes Apple the second most valuable company in America, behind Exxon Mobile (with a market cap of $278.6 Billion).

Personally, I don't think Apple is really worth that much.  Their Price/Earnings ratio is 20.8 to 1, compared to 13.5 to 1 for Microsoft and 13.6 to 1 for Exxon.  That means Apple stock costs about half again as much for each dollar of income as Microsoft or Exxon.  Apple's net income was $8.2 billion in 2009, compared to 14.6 billion for Microsoft and 19.3 billion for Exxon.

While all the stock analysts are claiming Apple stock is only going to go higher, I don't see this happening.  The numbers just aren't there.  Going by income and P/E, Apple should be worth about $125-150 billion, about on par with Google.

But never underestimate Steve Jobs and the hype surrounding him.  When he dies, Apple's stock is going to absolutely tank.