The quants over at Silicon Valley Insider have been doing some math in their spare time, and they came up with a great observation: Apple is about to sell its ten millionth iPad, if the company hasn’t already done so. When the company does cross that number, it will be a significant milestone considering the device first went on sale April 23rd, less than eight months ago.

So the math is pretty simple: apple had sold 7.5 million iPads through the September quarter. SAI used Gene Munster’s estimate of 5.5 million units for the December quarter and on the assumption that most of those iPads will be sold the closer we get to Christmas, that means we should be getting close to the 10 million mark.

Mr. Munster’s numbers are towards the high side of the middle. For instance, Mike Walkley of Canaccord Genuity just recently raised his own estimates from 5 million to 6.3 million units on the quarter. Other analysts range somewhere in the 5-6 million range. In other words, for back of the napkin math, Mr. Munster’s numbers are as good as anyone else’s.

In the meanwhile, 10 million units of a new product category is a big deal. It took Apple more than a year to sell the first ten million iPhones, for instance, and a bit longer to sell ten million iPods. It remains to be seen if Apple is excited enough to make an announcement, and that’s assuming it didn’t already hit that number earlier in the quarter.