The following chart uses comScore data to show the relative consumption of Android vs. iOS by the subscribers of the four major US operators. I modified an original chart published by Silicon Alley Insider.

A few observations:

As of November, the ratio of iOS to Android users was more than 15 to 1 at AT&T.

iOS at AT&T has twice the users as Android at Verizon

Although T-Mobile had the Android franchise to itself for all of 2009, it was overtaken by Verizon within four months

T-Mobile Android consumption has also been nearly matched by Sprint within a year

The rapid eclipse of T-Mobile smartphone penetration makes one wonder how effective Android is as a differentiator for carrier and device vendor alike. It also explains their new pricing model.

The data also gives clues to how iOS at Verizon will grow.

Since Verizon and AT&T have nearly the same number of subscribers, excluding RIM ,which is common to both, AT&T has a far larger penetration of smartphones (16.5 million vs. 7 million.) Is the cause iOS itself or the delay with the timing of Android launch at Verizon?

Other data from comScore shows that much of the gains for Android at Verizon have been at the cost of RIM, so Android has not been entirely additive. Will there be a similar platform switching going on or will iOS be additive to the Android base?

On one hand, the potential exists for a similar number of touch smartphone users at Verizon as at AT&T (15 million as of November). On the other, Verizon is “half-penetrated” with Android. How many will switch (as they did with RIM) and how many will be new users?

I’m not going to make any predictions yet but I would not be surprised if Android at Verizon will flatline if not drop while iOS at Verizon will jump quickly to make the sum of both equal to AT&T iOS in about twelve months. That would imply about 10 million new Verizon iOS users by 2012.

If Verizon users start to switch from Android to iOS, iOS could add as much as 15 million users. It seems that 10 to 13 is a reasonable estimate.