Great guys, glad they were of use.

@Heelx: Unfortunately you can only render images of a certain size with the unregistered version of Terragen (and the registered version is $99)! So the maximum square power-of-two size you can render is 512x512. So I can't render them any bigger... too bad, I too would like to have them at at least 1024. In my opinion the biggest problem area (i.e. blurry/boxy) is the terrain, so I'd recommend just covering it up with a scene map if you need to. And if the clouds are too blurry for your taste, I'd also recommend adding a low-alpha sky dome texture layer over them, to spice them up a bit. (I recommend doing this anyway--it really brings them to life. See readme.txt.)

And yes, there is a tutorial for using Terragen for skyboxes on Acknex Unlimited. It's by Steempipe, and it's for A6, but it should work just fine. My addendum to his tutorial is that you have to rotate the 5th bmap 180 degrees before making a panorama.
EDIT: Shoot, I can't find the tutorial there now. I'll upload it in a second.

I'd encourage anyone who is impressed by these to download Terragen and work on some of your own. Just keep in mind that the maximum height for your skyboxes is going to be a 512 unless you shell out the $99.