In reply to AndyCook:
Snowholes are pretty safe:- unless you dig one in the middle of some vast avalanche prone gully. or perhaps under the biggest cornice you can spot.
There is a risk - a small risk of the snow hole roof collapsing. But the normal practice is to mark the top of the snow hole with crossed walking poles/skis etc., etc., to prevent folk walking on the top of it. If you are on a steep/er slope this is unlikely to happen. A good foot or more of solid snow in a domed roof should support someone anyway.
Snow hole roofs don't collapse - in warm, thawing conditions they are more likely to start to sag. and sag, drip, melt and so on.
Don't forget snow is an excellent insulator. it isn't exactly easy to melt a snow hole even when a couple of you are using stoves.
Ventilation? Common practice is to keep a little bit of the entrance open during the night and/or stick an ice axe sized hole through the roof.
I've always made benches and seats inside once the main hole is done so you can sleep on - this stops your mates from standing on you during the night. It also keeps you away from the coldest layer of air.
As a real refinement you extend the hole and make a loo to avoid going outside.
If there are two of you it is a simple matter to dig two separate holes and once you are deep enough into the drift then both turn and dig towards each other. Then block one entrance up once you're through shovelling snow.