In reply to Cadairmanuk:
I haven't done Striding Edge in winter, and I haven't done Crib Goch at all. I should think the arete is about as exposed as Striding Edge. It may be a bit longer.
You say you've done Scottish winter II/III. The CMD arete is grade I. On that basis you should be fine. Don't even think about it without axe and crampons, but if you have II/III experience I'm sure you know that.
If I was doing it as things are right now, my main three concerns would not include the arete itself at all. They would be (i) avalanche potential getting up to the summit of Carn Mor Dearg, (ii) avalanche potential on the big slope between the arete and the Ben Nevis summit plateau, and (iii) navigating off from the top of the Ben if the weather is thick--again there will be avalanche potential whichever way you go--watch out especially for the Red Burn.
But enough doom and gloom. Do it when it looks right, and you'll have a brilliant time.