I tried using a picture of Hubbles Deep field as the starfield in sins and it looks like I'm wearing telescope lenses for glasses, but it certainly livens the backdrop up a little. Another good source for making starfields is google earths Google Sky mode, there's a lot of very highly detailed high resolution star clusters and nebulas on there but I don't have the first clue how to make the textures seamless, they all seem to have a visible banding where the textures are joining so I'm hoping some of the talented people on here make some soon.

Here's a link to the file if you want to try it just drop it into the "C:Program Files\Stardock Games\Sins of a Solar Empire\Textures" folder, be sure to backup the original stasfield.dds first so you can change it back if you want to.
http://rapidshare.com/files/95246309/starfield.dds