Joining the growing list of analysts who have “baked in” the launch of a Verizon (VZ) iPhone in the first quarter of 2011, Piper Jaffray’s Gene Munster offered two sets of estimates Wednesday:

• A “conservative” scenario in which Verizon sells 9 million iPhones in 2011 at the expense of AT&T (T), which sells 4.6 million fewer than it sold in 2010
• A no-cannibalization scenario in which AT&T continues to sell iPhones at its current rate and Verizon sells nearly as many in 2011 as AT&T did in 2010.

The conservative scenario is the one Munster is using in his model, represented in the pie chart above and the table below the fold. He starts with an assumption that 80% of the smartphones AT&T sold last quarter were iPhones, despite its diverse portfolio of smartphone choices — iPhone, Google (GOOG) Android, Research in Motion (RIMM) BlackBerry, Hewlett Packard (HPQ) Palm, Nokia (NOK) Symbian and Miscrosoft (MSFT) Windows 7.

“However,” he writes, “we are conservatively modeling for the iPhone to represent a much lower share of smartphone sales at Verizon. We estimate that Verizon will activate roughly 25m smartphones in CY11, of which 9m, or 36%, will be iPhones.”

Below: Munster’s breakdown of international vs. U.S. iPhone unit sales for 2010, 2011 and 2012 under his conservative scenario.

Note that 66% of the iPhones Apple (AAPL) sold in calendar 2010 were purchased outside the U.S., a share Munster expects will grow to 68% in 2011.

See also:

• How many iPhones will Verizon sell next year?

[Follow Philip Elmer-DeWitt on Twitter @philiped]