UPDATE: With the announcement Thursday that Apple had sold only 450,000 iPads, Chitika hastily reset its counter and issued this apology:

“The good news is that Steve Jobs has given us (and the rest of the world) a second data point – about 450,000 iPads sold through end-of-day yesterday.  With this, we’re able to get the proper assumptions – mostly, we need to refine our assumptions on how many unique IP addresses the average iPad hits our network from. We should have the tracker updated with the second data point and correct calculations soon.  Obviously, our 700k number is fairly far off.  Thanks for the patience.”

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With so many estimates of iPad sales floating around — from Apple (AAPL) and from everyone else — we thought we’d offer a snapshot of a different kind of number.

It’s from Chitika Labs, a search-targeted ad network that claims to serve more than 2 billion ads per month. The company put up a nifty widget Monday that estimates how many iPads there are in the wild based on the number it sees passing through its advertising network.

As of Wednesday at 5:15 p.m. EDT, Chitika’s count was up to 527,315. Click here to get their latest.

In addition to the big number ticking away like the seconds on a digital clock, Chitika has posted fever chart showing sales per hour and a map of the U.S. showing the states with the most iPads (California, Texas and New York top the list) and iPad density per population (the District of Columbia and Hawaii head that list).

The numbers are based on what we assume are a relatively small sample, so take them for what they are worth. Moreover, Chitika’s estimates are running considerably behind both the whisper numbers and the official tally, as Chitika’s Dan Ruby explains.

In a blog post Monday, Ruby  pointed out that even though Apple announced sales of more than 300,000 units as of midnight Saturday, his count a day and a half later was only 270,000. Either some of those iPads are still in their shrink-wrapped boxes — perhaps for resale overseas — or they still haven’t gone online. (Ruby’s post via MacRumors.)

See also:

• Predicting iPad sales: How close were the analysts
• New iPad launch-day estimate: 600,000-700,000 sold
• iPad week two: 240,000 pre-orders
• How many iPads will Apple sell?

[Follow Philip Elmer-DeWitt on Twitter @philiped]