Broadpoint AmTech analyst Mark McKechnie’s estimate that Motorola (MOT) sold 100,000 Droid smartphones last weekend has been getting a lot of attention, although nobody’s quite sure what to make of it. McKechnie called the number “encouraging.” Nielsen’s Roger Entner found it “a little troubling.” IDC’s Ramon Llama said it was “nothing to shrug off.”

Part of the problem is that everybody is comparing Motorola to Apple (AAPL), which sold 270,000 iPhones in its first two days of sales in 2007 and 1 million iPhone 3GSs in three days last June. The consensus on the Street is that Motorola will do well to sell 1 million Droids by the end of the year.

The other problem — and the reason Nielsen’s Entner is so troubled — is that the ground had been softened for the Droid by a carpet-bombing ad campaign, the biggest in Verizon’s (VZ) history. According to Ad Age‘s Rita Chang, the carrier has budgeted $100 million to support the Droid, most of it to be spent before the end of the year.

You can do the math.

How can Verizon afford to spend $100 per sale for a $199 (after $100 rebate) phone it is already subsidizing to the tune of hundreds of dollars apiece?

My theory is that the real purpose of Verizon’s campaign is not to attack the iPhone or even to sell Droids. The carrier’s true enemy is AT&T (T), to which it has been hemorrhaging subscriber share ever since the iPhone arrived.

Note that the iPhone only appears briefly in the “there’s a map for that” TV ads that targeted AT&T’s nationwide coverage. Verizon’s latest TV campaign is even gentler, as if the iPhone could leave the island of misfit toys if it only had a better 3G network.

“Makes sense if you want the iPhone to be on your shelves one day,” says a former advertising executive who watches Apple closely. “Push the Droid (without comparing it to the iPhone), but push your network as better than AT&T, and hope you gain enough traction with it to help persuade Cupertino that coming on board would be a good thing.”

The contract that made AT&T the iPhone’s exclusive U.S. carrier is reported to be expiring in 2010. According to AppleInsider, Apple has already signed up Taiwanese suppliers to build a hybrid “worldmode” iPhone that would run on Verizon’s network.

[Follow Philip Elmer-DeWitt on Twitter @philiped]