We can see that the Wii has been doing a phenominal job of breaking into this generation and stealing the lead from Microsoft, outselling the 360's 22 months of sales in 10 months. I originally believed that Microsoft and Sony would end 2008 with approximately 30% of the market share each, leaving Nintendo with 40% market share. But these numbers are definitely colored by my 'holiday-sales glasses' that see Sony and Microsoft doing much better than usual, but Nintendo continuing to do what they do year-round: sell out.
Sales will undoutably slow for MS and Sony in the first half of 2008, just as they did in the first half of 2007. Remember the ~7 months where Sony was barely breaking 100k units per month in North America? Their sales won't be that slow in 2008, but they will definitely slow down a lot. Microsoft did better, but not all that much better.
I believe that in 2008 the Wii will outsell the 360 and PS3 combined world wide, and I believe it will approach or eclipse 50% total market share. Let's look at some numbers:
Nintendo is producing 1.8 million Wiis per month right now, so we can safely assume they can at least continue producing that many through 2008, demand permitting. And it's unlikely demand will slack early enough in the year that they'd slow production for the holidays. This means, at the minimum, Nintendo will be capable of selling 21.6 million additional units from Jan 1 2008 to Jan 1 2009. Neither Sony nor Microsoft have come close to having a 10 million unit year. 2007 will be the best year so far for each of them, and assuming their sales climb incrementally in 2007, MS will probably have a ~9 million unit year and Sony might have a 11 or 12 million unit year. That would put Nintendo very close to 50% total market share.
Part of Sony's success in 2007 was due to the Eu launch which boosted them nearly one million units -- that's comperable to a month of their sales during the holidays season and it's a one-million unit spike they won't see in 2008.
It i s also likely that Nintendo will do significantly better than 21.6 million units. Even if Nintendo only increases their supply for the next holiday season, it isn't unreasonable to estimate that they could produce 24 million or better in CY 2008. This is a pretty tiny increase in available units (10% more than current production levels), and certainly managable if demand finds no slack in the first half of 2008.