A lot has changed since Needham’s Charlie Wolf published his last Apple (AAPL) valuation back in February 2010, and in note to clients issued Monday he rattles off the things that have surprised him most:

• Explosive iPad sales. “We forecast that iPad sales would reach 20 million in 2019, the final year in our valuation exercise. iPad sales are already tracking over 12 million annually even before a full international rollout. Meanwhile, the device remains in chronic backlog.”
• A shift in the Mac’s trajectory. “Mac sales are tracking to 14 million units in calendar 2010, more than 50% higher than the number we previously forecast for the year.”
• Rapid iPhone upgrades. “The recent iPhone 4 launch indicates that current iPhone owners are upgrading much faster than we previously assumed. This will increase total iPhone sales even as the installed base of iPhones grows roughly in line with our previous forecast.”

Wolf has a model of Apple that assigns a value to each of the company’s businesses — the iPod, iPhone, iPad, Mac, software and music — and in which growth in Macintosh sales depends solely on the halo effects from the iPod and iPhone. What that model tells him is that Apple’s shares should be trading at $375, not his previous target of $280 or Friday’s closing price of $260.09.

“In our view,” he writes, “Apple’s market capitalization has the potential to exceed our estimate. That’s because Apple is a small fish in three very large ponds.”

The three ponds: the worldwide PC market (Mac has 4+%), the smartphone market (iPhone has 14%) and the tablet computer market (for now the iPad has it all to itself).

Below: Wolf’s valuation model.

Note that in Wolf’s spreasheet, the Mac’s contribution to Apple’s share price has grown 72.8% faster than he expected while the iPad’s has grown nearly 250%.

But Wolf is pretty conservative when it comes to predicting Apple’s sales. He’s the only analyst we know whose revenue estimate for the company’s fiscal fourth quarter, $17.87 billion, is below Apple’s own guidance ($18 billion).

See also:

• Who buys Apple’s $18+ billion quarter?

[Follow Philip Elmer-DeWitt on Twitter @philiped]