A splash screen's main use is to tell a user that the program has begun loading, and specifically to inform them that that they dont need to run the program again and waste time.

A good splash screen will tell the user the name of the software, the vendor's name, the vendor's website address, the user's registration details/a registration reminder and program version info including service packs (Microsoft). It is also a bonus if, as WWWWolf mentioned, it shows items being loaded, so savvy people can find where things are going wrong. I have found that many users appreciate good info in the splash screen.

Do make an effort to have a good image for your splash screen, the GIMP (again) has a good image, as a bad image/plain text will make users subconciously think that your program isn't very good.