Total iOS devices: 315M (as of 2012-03-07) Total Andriod devices: 300M (as of 2012-02-29)
http://techcrunch.com/2012/03/07/tim-cook-talks-ios-device-s... http://www.guardian.co.uk/technology/appsblog/2012/feb/29/an...
Android is selling faster, but it has yet to have a larger installed base. It will likely happen soon, but iOS had a large lead in terms of start time and the iPad is killing in the tablet market.
Obviously there are many factors which will adjust the true installed base (broken devices, devices no longer update-able, etc.).