At the time of writing - 7 minutes past my lunchtime on the frosty 1st of December, 2010, I thought I would gently remind the world that you can never plant anything in frozen soil.
It's the roots, you see. A root is an amazing thing, but it can't do everything at once.
When it's frozen, it loses all of its elastic strength and becomes quite brittle. If it is moved around while frozen, it will crack and split and die.
The other thing to watch out for is this: if you order bareroot plants during winter and you need to store them while a frozen period passes, they should be stored outside, where they can freeze, out of the sun.
If they freeze all night and then quickly thaw when the sun hits them during the day, the roots are likely to be damaged.