I doubt it's because of their US situation - that has been really bad for a couple of years now. And Nokia never really had any market share among CDMA phones to start with, so there's nothing there to lose.

I think it's simply because their Q3 lineup is a bit lackluster - they haven't really introduced any significant new models that became available prior to Q3, had issues with the 6220 Classic (I assume that's the mid end device they mentioned in the press release). They lacked some new stuff during Q3 - the E71/E66 are very nice, but only now reaching the market via operators here, the N78 is still too overpriced for its functionality to reach more people I'd say, and upcoming cash cows like the SuperNova lineup and the 6600 Slide/Classic are only starting to become available now.
Combine that with a very aggressive and impressive Samsung, a very successful HTC and Apple's iPhone 3G, and I think it's not surprising to see a market share dip of a couple of percents.

And at almost 40% of such an extremely competitive market, basically the only way is down anyhow if you ask me.

In the end, if things go a similar way as with their 2004/2005 crisis, then I'd say Nokia will come back better than ever after that mini-crisis at the moment