The chart above is from (unfortunately) the last AdMob report on phone trends.  It is built based on the number of times each OS made a request to AdMob, which until Apple’s iAds get moving, is a fairly accurate measure.  The entire report can be accessed here.

You can look at this chart two ways.  From the first perspective, Apple still has a 50% lead on Android, and this is before the iPhone 4 release.  From the other perspective, there’s only one platform with consistent momentum over the course of the last 12 months, Android.  This is going to be the case for as long as it takes Android to catch Apple (probably 18 months give or take 6 months).  Apple’s chart will continue to be spiky because it relies on a single set of hardware.  A single set of hardware means sales will be closely linked to new releases.  Android on the other hand will continue a steady rise, at an overall faster pace.  New Android phones come out every other week or so (informal guess).  They aren’t as big of a deal when they do, but it’s a tortoise and hare debate.