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.

See also:

• The wild Apple iPad ruckus begins
• Apple hits another record high
• Apple approaches $200 billion
• Apple sets iPad ship date; shares hit record high

[Follow Philip Elmer-DeWitt on Twitter @philiped]