Shares surge on iPad pre-orders, making Apple America’s third most valuable company

UPDATE: That was short-lived. By March 15, Apple’s market cap had fallen below Wal-Mart’s once again.

Ninety minutes after Apple AAPL opened its online store for iPad pre-orders Friday morning, a burst of trading — roughly 2 million shares in 30 minutes — pushed its stock price to $227.73 and its market cap to $206.5 billion.

That, and a dip in Wal-Mart’s WMT share price, made Apple — if only briefly — America’s third largest company by market capitalization, after Exxon XOM and Microsoft MSFT and ahead of Wal-Mart and Berkshire Hathaway brk.a .

Apple shares have been on a tear since the iPad’s ship date was announced last Friday, hitting five all-time highs in as many days.

UPDATE: Apple closed at $226.6, up 1.1 points (.49%) for yet another all-time high. It’s market cap is now $205.48 billion, $309 million more above Wal-Mart, which closed down .07 points (.13%) for the day.

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[Follow Philip Elmer-DeWitt on Twitter @philiped]