Apple (AAPL) is back in the $500 billion club, to use a phrase employed by Standard & Poor’s senior index analyst Howard Silverblatt, who yesterday circulated a note to clients stating that Apple would need to trade above $537.98 in order to have a market cap above $500 billion.

Now, you could certainly do the math yourself, as Apple’s most recent 10-Q filing with the SEC lists 941.57 million shares, which, dividend into $500 billion, equals $531.

But Silverblatt is accounting for the vagaries of different share counts between different data sources. So, he says $537.98 or higher is where Apple needs to trade.

Today, the shares are up $10.36, or almost 2%, at $545.77.

Silverblatt offered some interesting stats to think about.

The 2010 data from the International Monetary Fund showed 19 countries with a GDP higher than $500 billion.

And today could be a history-making close for Apple shares.

If Apple closes this day at $500 billion or great, it would be only the sixth issue in history to close any month with that great a market cap (leaving aside inflation-adjusted numbers). The others are Cisco Systems (CSCO), which hit $538 billion back in March of 2000 (how times change!), Exxon Mobil (XOM), with $513 billion back in July of 2009, General Electric (GE), with $581 billion back in August of 2000, Intel (INTC), with $503 billion in August of 2000, and Microsoft (MSFT) with $604 billion in December of 1999.

And Apple one of only 11 companies that have reached $250 billion in market cap. The others are AT&T (T), Cisco, Citigroup (C), ExxonMobil (XOM), General Electric (GE), Intel (INTC), Microsoft, Oracle (ORCL), Pfizer (PFE), and Wal-Mart Stores (WMT).

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