Inasmuch as Microsoft (NASDAQ: MSFT) isn’t saying much about Surface RT unit sales to date, the vendor has left itself wide open to speculation, estimates and educated guesses about how the tablet is doing with consumers. While mid-December financial analyst and market research estimates pegged expected Surface RT sales in a range from 600,000 to 1.3 million units for Q4 2012, now–just shy of three months since the product’s debut–we have yet another run at it. Here are the details …

The latest projection comes from UBS managing director Brent Thill, who, in a note to customers, halved his Q4 2012 forecast for the device’s sales from 2 million units to 1 million, blaming strong competition from Apple’s (NASDAQ: AAPL) iPad as well as Microsoft’s limited distribution scheme as the culprits in his downgrade.

Thill’s estimates fall somewhat between iSuppli’s sales forecast of at least 1 million and as many as 1.3 million Surface RT units and Detwiler Fenton’s projection of 500,000 sold tablets for Q4 2012. The Boston-based brokerage house, incidentally, estimates sales for the impending Surface Pro in the 2 million to 3 million range.

Similarly, Microsoft’s modest Surface RT prospects haven’t dulled Thill’s enthusiasm for Surface Pro, about which he is projecting some 2.5 million units will be sold in 2013, according to report published earlier this week. At this point, Microsoft hasn’t said whether its channel partners will be able to get their hands on Surface Pro, but one would think if the device is positioned as a laptop replacement, the vendor’s distribution blueprint will be broadened beyond what it enacted for Surface RT.

Late last week, Samsung cancelled production of its Windows RT tablet, dubbed Ativ Tab, after it received a lukewarm reception from the vendor’s retailers and the Korean manufacturer couldn’t settle in on an acceptable go-to-market strategy for U.S. sales.