Megan, good question. In a traditional greenhouse, you usually don't plant directly in the soil, but I suppose you could if you don't have a cement floor inside. It would depend on your greenhouse. Also, most of the larger greenhouses are not very affordable for the average homeowner.
An alternative would be a hoop house constructed from pvc or electrical conduit and perhaps some wood, covered with a gardeners grade plastic material. It's MUCH more affordable. I have a video on my blog this week that shows one example and links to another. You might want to check it out or google "hoop house" and see what you find.
If you want to get really fancy, check out this video where Eliot Coleman talks about his and how he moves it depending on the crop and season. (http://www.youtube.com/watch?v=omMZBXo53_M)