Most is just speculation now, but people seem to think that egg spawn-rates are dependent on the PokeStop, rather than just a general chance like 5% 10km, 10% 5km, 20% 2km or something like that (in this example, percentages don't add up to 100 because there isn't a 100% chance you will get an egg from a PokeStop). Read more here. Most who are speculating estimate the chance of getting a 10km egg at a PokeStop (if you have open egg slots) to be ~4%. One user, who has reported 280 eggs hatched, has stated that around 1/40th (2.5%) of his/her eggs were 10km eggs.
Players have reported different experiences, with some Level 20 users reporting no 10km eggs and some Level 3-5 users reporting multiple 10km eggs. From my own experience and some research on the interwebs, it seems that PokeStops in more populated areas seem to drop more 10km eggs, or have a higher likelihood of doing so.
For example, I live just south of San Francisco. In my local town, I've only gotten one 10km egg, but when I've driven up to downtown San Francisco (into the city), I've earned a total of five 10km eggs.
There is also an ongoing research effort by serebii.net forums to try and take user data to predict egg drop rates. It's in an early stage and could benefit from more contribution, but the current results can be found here.