Android development: api 14 vs api 15
Less than 0.05% of devices are on 4.0, 4.0.1, or 4.0.2, which is why it doesn't show up on the dashboards. Similarly, less than 0.05% of devices use 2.0 or 2.0.1, and same with 3.0.
You can safely develop for APIs 8, 10, 13, 15, 16, 17, 18, and 19 (as of Dec 28, 2013), and forgo the remaining nine API levels (though there is seldom a reason to). If possible, you may want to continue to support API 7 as well, since it just recently went off the dashboards. (0.05% of a billion devices is still quite a few!)
Google has indicated to manufacturers that android 4.0.3 should be used as the base version for ics. It also has recommended all manufacturers to provide automatic updates to 4.0.3.
I have no idea about usage statistics of devices running versions before 4.0.3, but I guess it will be a minority of the devices.