Well, one has to be a little careful on this subject. In general, I think candle lanterns are safe in a tent, and I’ve used them often in that situation. Particularly for winter camping, a candle lantern puts out quite a bit of light, and if the wind is not blowing hard, can raise the inside temperature of a tent by ten to 15 degrees. A good buy in a single-candle lantern is the REI Aluminum Candle Lantern ($18). For some real light output, the Uco Candelier ($34) can burn one, two, or three candles.
That said, you’re talking about an open flame (well, aside from the glass tube around it) and that can never be as safe as a flashlight. Sure, you can hang the lantern, or set it on the tent floor away from the walls, but there’s always the risk it gets bumped or something. Modern tents all have to pass flame-resistance tests, so the risk of a sudden “whoosh!” followed by a puzzled look on your face as you sit with charred eyebrows amid the tattered remains of melted nylon and some smoking tent poles is low. But you could put a hole in it, or damage a sleeping bag or other pieces of equipment. Of course, any combustion yields carbon monoxide, so you want to be a little cautious and make sure you have some ventilation in the tent. And, don’t go to sleep with it burning!