Nokia's Pekka Pohjakallio told us last week in Barcelona that adding Windows DRM was the primary cause of the slip. That confirms what Nokia executives said when the delay was acknowledged last Fall. Nokia had originally hoped to ship the N91, which was announced back in April, by Christmas. Instead, it promoted the N70, an incremental improvement over the 6680 series, as its Xmas bait. The N91 is due by the end of Q1.
Supposedly, DRM is supposed to make things on manufacturers and retailers easier, not harder. The exact reasons behind the delay aren't clear, only that DRM was involved, so the truth could be a whole number of things. Perhaps Nokia will reveal more details as the next “launch” date approaches.