With an estimated 8.5 million units sold to date, Apple's iPad is now the best-selling tech gadget in history, according to a financial analyst who tracks sales of the device.

Slideshow: iPad Does Remote Control--3 Apps Tested

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Colin McGranahan, of Bernstein Research, called the iPad "a runaway success of unprecedented proportion" in a research note to investors this week.

The iPad smashed industry records earlier this year when it sold 3 million units in its first 80 days on the market. Third quarter sales were in the range of 4.5 million units, and overall it's estimated that Apple has shipped about 8.5 million iPads to date worldwide.

The iPad's blockbuster sales rate is beginning to eat into PC sales, partly at the expensive of Apple rival Microsoft. Research group NPD last week released a survey that found 13% of iPad buyers would have purchased a PC had the iPad not been available.

In a further boost to what Apple CEO Steve Jobs calls "a magical device," the iPad hit Target stores throughout the U.S. on Sunday under a partnership that's designed to introduce the personal computing tablet to a larger audience while boosting foot traffic at Target's brick-and-mortar outlets during the crucial holiday shopping period.

Target is carrying five of the six iPad models Apple produces, and is selling them only in-store, not online. Available are the 16GB Wi-Fi and Wi-Fi + 3G models, the 32GB Wi-Fi only model, and the 64GB Wi-Fi and Wi-Fi + 3G models.

Target officials said they expect the hot-selling iPad will be popular with its consumer base, which tends to be less tech-savvy than individuals who shop for tech gear at dedicated electronics outlets.

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