As Fortune points out, Nokia is the dominant smartphone maker and the dominant handset maker in the world, but it continues to struggle in the US market. The article argues that Nokia's success formula outside the US has limited its prospects in the US:

The problem is not the product. The n97 might not be ready for sale, but Nokia's e71, which looks like a sleeker version of the BlackBerry Curve, has won design awards and dominates many European markets. And it has several phones in its n-series of multimedia devices that boast the best cameras and videocams in the biz. North American president Mark Louison says U.S. carriers will soon support the phones. But ask the phone companies, and they say they have no plans to roll out Nokia smartphones anytime soon.

By lagging in smartphones Nokia isn't just missing out on sales; it may also be losing the attention of software developers that make cool games and applications for mobile devices, a growing number of which operate in the U.S.

Nokia is reinventing itself yet again as an Internet company, a sort of Yahoo.com for your phone. It is trying to woo application developers to its mobile platform through offices in Silicon Valley and Boston. You'd think it would be tantalizing to write software for the world's largest mobile platform. But ask developers worldwide to show you their favorite mobile apps, and they'll probably pull out their iPhones.

The North American market has become the dominant mobile Internet market -- and the most competitive -- in a very short period of time. 

The AdMob data show how quickly the iPhone has encroached on Nokia's smartphone share:

Here's a closer look at the US smartphone market; Symbian is flat at less than 5%:

Although the Fortune piece argues that devices/handsets aren't the problem Nokia needs to excite people with a handset in the way that the Pre has appeared to rekindle Palm's fortunes in one stroke. Perhaps the N97 is that phone, although its suggested price €550 ($695) will likely prevent it from becoming mainstream vs. cheaper competitors in the US market.

Failure to gain a larger share of the US and North American market means eventual share losses in Western Europe and possibly elsewhere.