First of all, we don't know if 150,000 devices have been sold. That was an estimate. I am not sure what the error bars are on that estimate, but it wouldn't surprise me if it was 10,000-20,000 units off either way.
Secondly, there are only about 30 apps in Palm's Pre app store. There are some very good ones, like Evernote, which should have broad appeal, but there are others that will have more limited appeal, like Fandango (I refuse to pay $1 per movie ticket for the privilege of buying a ticket online) or FlightView, which would appeal mostly to travelers or pickup services.
To arrive at the million download mark, Medialets assumed that the 150,000 figure was accurate and concluded that, on average, every Pre owner was downloading seven applications. But in reality, neither figure may be correct, or even close.
I say we wait until Palm releases some actual data on both sales of the Pre and downloads from their application store. Such a release may validate these estimates or show them to be way off, because right now, we don't know how many devices have sold, how many apps each user is downloading or how much in total have been downloaded. But, it makes for good press releases.