Via Tom Ricks at the Best Defense, here's an interesting discussion of the practicality of this. Bottom line, it's probably not practical for container ships, because they're limited in draft and beam, even though it can be significantly faster.
I recommend a book called The Box, on the development of containers. This quote from the article is significant:
What is far more important than speed is reliability. Unlike the bulk shipping discussed earlier, schedule integrity is a key service-attribute for containerships. The Arctic will always suffer from periods of poor visibility and the potential for wind-driven ice, both of which can make routes with a comparatively low average transit time have a large variability around that average. More than half of all container cargo is now component-level goods—materials destined for factories for use in production processes operating on a just-in-time-type inventory-management system. That makes consistency, reliability, and schedule integrity of paramount importance. The key goal of container shipping is 99 percent on-time delivery. If this is attainable at all, it will be extraordinarily expensive using Arctic transit routes. Thus the variability in transit time that may be tolerable in bulk shipping is unacceptable for container shipping.