Somewhat wrong title, it is the iPhone market share of smartphones that have increased to 10%. Smartphone sales represented only 13.5 per cent of all mobile device sales in the first quarter of 2009. So apple's share of the overall market is still less than 1.5%.
The whole article is very confusing regarding this, mixing the total market and smartphone market without giving any hint as to what it means with the particular numbers. Very badly written.
And it's not market share of "phones in active use", it's market share of "phones sold during the quarter". And it's not compared to the market share of phones sold last quarter, it's compared to the market share of phones sold during the first quarter of last year.
In that time, we've seen the iPhone 3G release, a rollout to many more countries, and a big advertising push, which no doubt helped their market share immensely.
Data from the original report http://www.gartner.com/it/page.jsp?id=985912