I read a NYTimes article about people actually having "earthen floors" (= mud) installed into their homes. Crazy (in my opinion) but anyway, it mentioned that they are finished & sealed with some sort of wax.

Here's one result from googling:
http://www.treehugger.com/files/2007/02/track_that_mud.php

It appears linseed oil may be the solution. Read on:

http://www.landerland.com/efloor/efloor_inst.html

I have the same situation: perfect trees, great house, perfect place, dirt basement. My reno guy says the thing to do is lay down heavy plastic vapour barrier, in overlapping rows. There's a special tape called, I think, 'tuc tape' - NOT duct tape, I promise - that you then use to seal the joins in the plastic strips. My basement has concrete walls rising up from this dirt floor, so he's also told me about a special product used to seal the plastic/concrete join. Can't remembr what it's called, can find out if you want more info.

I don't know what your idea of expensive is. I don't think you should base a remodeling choice on a lack of information. All you need to do is call a few contractors and get a few estimates. I would recommend putting plastic and foam insulation down before the pour.
Concrete also keeps rats from coming in through the dirt. There's another post in this section with a dirt floor in a crawl space. They have and issue with water and rats.
Ron

If youÂre still interested, there is a product we use for parks and for side yards, access paths. called Polypavement. It is a soil stabilizer. Mix with water, non-toxic (smells like ElmerÂs glue), spray onto dirt floor let dry. One application can last for decades in a low traffic, low impact area.
http://www.polypavement.com/