I only know for the CDMA Hero, which I have. I used Pre-kitchen, which you run on your computer. You can root your phone with it and then it has an option to change the splash screen. Available with the program now are the original sprint, Nexus One, and another one. Again, this is on the HTC Hero, so I guess it depends on what phone you have. I'm sure there are options for almost all Android devices.
I'm also pretty sure you could get other splash screens that don't come with pre-kitchen, again, if you have the Hero. Sorry if this was no help but maybe I've led you in the right direction. Check these forums for rooting how to links and check over at XDA.