Hi there! Microsoft has come out with some numbers for Windows Phone 7. Microsoft says that every person on WP7 downloads 12 apps a month. Based on my calculations, WP7 should be around 190 million downloads. My way of getting it is:

12 dld/person * 3.5 million(estimated # of WP7 handsets sold to consumers) = 42 million.
42 million dlds/mth * 4.5 months.

This method obviously isn't accurate because there isn't any way of knowing what the rate was a few months back. A more accurate estimation is 120-150 million.