It's not so much the iPhone lost market share, it's just that other manufacturers made stronger gains in the quarter, but iPhone sales are still stellar and making Apple a healthy profit... IPhone Loses Market Share in Fourth Quarter - Digits - WSJ
It's probably more to due with the fact that other brands do a Christmas release, while Apple releases the iPhone in the summer. I will say that Apple will probably have to find something to shake things up for the next release. Since it appears they're sticking with AT&T, there's going to be a limit on how many more 3Gs's they'll be able to sell. With an LTE version probably not coming for a couple of years (AT&T isn't planning an upgrade for a while), they'll have to do something to get people to upgrade again. Other than processor and more memory, there's not much else they can put into another version without some sort of big change to the look and feel.
Yes, it is true the iPhone market share dropped from 18% to 16.6% from Q3 to Q4 in 2009. But, the total sales from Q3 to Q4 increased by 18% to 8.7 Million phones. And comparing Q4 2009 to Q4 2008 (what is normally done), the iphone market share actually increased 10.8% to 16.%, selling 4+Million more phones between the two quarters. A different way of looking at it, and indeed, other surveys emphasis the Q4 to Q4 data more.