Just wanted to add that I didn't mean to say I wouldn't like this ability in Terragen as well but I guess it falls into the same sort of category as star fields. I think the ability to add a star field inside Terragen would be a nice adition as well, with the ability to set the amount of stars, the colour, and a random seed etc. The problem is that this can also be created outside of Terragen, but it's a hassle to have to load up another package and spend longer creating it, save it out, import it into Terragen, rather then just clicking a button from within Terragen. Of course assuming you didn't want exactly the same star field in all your images you would have to go through this entire process again to create another image instead of just hitting the random seed button.
I guess what I'm saying is that I tend to agree that these things would be a nice addition to Terragen but because they can be done (fairly) easily in other software they may be a lower priority than say SSS which everyone seems to want.