According to new numbers published by Gartner, Nokia's worldwide market share for smartphones has slipped from 51% to 41%. Research In Motion and Apple stole the biggest slice from Nokia. Why is Nokia losing out?Just two years ago, Nokia held more than 70% of the worldwide market for smartphones with its S60 platform. Since the introduction of more appealing models from competitors such as RIM and Apple, Nokia has seen its worldwide market share slip quickly.

Here's how the numbers break down. RIM nearly doubled its market share, climbing from 10.9% to 19.5%. Its sales grew at a rate of 85% and it sold 7.4 million phones. RIM can thank models such as the Storm and the Bold for its success.

Next up is Apple. It also doubled its worldwide smartphone presence, jumping from 5.2% to 10.7%. Unit sales in the fourth quarter surged from 1.9 million to 4.1 million devices.

HTC and Samsung also made gains on Nokia. HTC grew from 3.7% to 4.3%, and Samsung grew from 1.8% to 4.2%. (Motorola is nowhere to be seen.)

Nokia still beats RIM, Apple, HTC and Samsung combined, but that's changing. If I were Nokia, these numbers would make me nervous. It has an exciting lineup of hardware in the pipe if not already on the shelves, so why are users defecting to other smartphones?

It has to do something with the past-its-prime nature of S60 and Nokia's continued inability to make a dent in the North American market, where Apple and RIM reign. The fact that it just got an iPhone-competitor to market in the 5800 XpressMusic hasn't helped.

Nokia will no doubt remain the No. 1 player for some time, but that doesn't mean it can rest on its laurels. With the erosion of its worldwide market share of smartphones dropping so precipitously, it has to attack the competition head on.