In reply to squicky:
Quite a lot of info, including helpful links, has been posted in several UKC threads over the last couple of years, so a thorough search would be well worthwhile.
In Reykjavik on 31st December, sunrise will be at 11.21 and sunset at 15.41. This may be one of the reasons the guiding companinies aren't doing trips!
Also, bear in mind that Iceland enjoys the Gulf Stream, so conditions can be fickle. I've arrived in Reykjavik in winter to find 6" of snow on the ground and daytime temperatures at –10C with a strong N wind, then a few days later, I've left in pouring rain with snow almost gone and temps of +10C. Last year, there didn't seem to be much snow in Reykjavik at all, though the Highlands get lots of snow – but the problem with that is access...
Post edited at 15:57