According to a new report from research firm NPD, iOS’s U.S. market share (by sales) jumped from 26% in the third quarter of 2011 to 43% by October and November. Android, however, came out on top, with 47% market share during those two months, down from 60% in Q3.

Says NPD, over the course of 2011, the smartphone battle saw iOS and Android distancing themselves from the competition, turning it into “a two-horse race.”

Overall, smartphone sales continue to grow, NPD says, and accounted for two out of every three handsets sold in October/November 2011. For comparison purposes, that’s up from 50% in Q4 2010.

In addition, nine of the top ten phones sold in Oct/Nov are smartphones, with Samsung, HTC and Motorla each having at least one smartphone listed in NPD’s top ten. Apple’s phones are on the top of the list, with the iPhone 4S in the number one spot and the iPhone 4 still doing well in the #2 position.

It’s clear that iOS and Android are dominating the market here, as former leader RIM’s sales tanked from over 20% in Q3 2010 to just 6% in October/November 2011.

We recently got a glimpse of the extent of iOS and Android’s growing popularity, when a previous report from mobile analytics firm Flurry from December had confirmed the massive gains those platforms were making over the 2011 holiday season. The firm found that 6.8 million Android and iOS devices were activated on Christmas Day alone. That was up 353% from the 1.5 million activations a day over the first 20 days of December. It also set a new record: Christmas 2010 had only seen 2.8 activations, Flurry said.

We’ve also been watching Android’s steep market share climbs based on activations – Google’s Andy Rubin tweeted in December that there were now more than 700,000 Android phones activated every single day, which is up from 500,000 activations per day last June. Mobile analyst Horace Dediu at Asymco then estimated that the total cumulative number of Android devices activated so far is between 224 million and 253 million. To compare, last October, Apple announced a cumulative total of 250 million iOS devices sold (including iPods and iPads).

For what it’s worth, research firm Chitika was also reporting today that Android was leading in market share with a 51.6% share versus iOS’s 46.4% share. But this data was based on web impressions from the last week of November and December. That’s not enough data for anything but a quick snapshot of traffic levels during one of the busiest times of the year – holiday activations.

We won’t know for sure how many iPhones Apple is selling until its next earnings report, but thanks to NPD’s new data, we know it’s going to be neck-and-neck.