I made a custom sky for our new map DM-Cyclopean_SE and I will also be making more as I enjoy creating custom skies. I know it is not the best but at least it/they will add some new flavor to your map.
If any1 is familiar with PhotoShop you can edit it to your liking to use in your own map. Adjust the brightness, colors etc... then save it and import your new sky to your map. It is nearly at the highest resolution(except for GI?) I could do with the free version of Terragen 2.< which is an awesome free program. Check out their site. http://www.planetside.co.uk/terragen/
edit:This sky is only previewed as 1024X256. They must be 2048X512. I do not know why PhotoBucket makes them smaller and I do not know how to get around that. I will make a pack soon and upload them all to FileFront. Plus PhotoBucket transferred it from a .bmp to a .jpg. Sorry but will have a pack for every1 soon.
Here is an awesome tutorial on how to apply the sky to your map. http://outpt.co.uk/tut012.php In this tut you may also download a template to use to make your original skydome(which I thought was extremely nice of them).
Also DGUnreal has awesome custom skies here http://www.lilchips.com/dgunreal/ut3-skies.asp with a very nice tut as well.
This particular skydome is ready to the point of just importing it in your map.