I think (although not sure as I never have done this) you have to use a generic RUU to debrand it. This would replace the splash screen.
The rooting method I used way back when - did this and removed all my branding.
If you rooted an existing rom say using unrevoked, that wont get rid of it.
You can debrand by using the RUU flashing method, but you'd have to flash a 2.1 ruu to make it easy to root it (as you would need to do this again.)
Hope this helps.
Please wait for other answers before doing anything though because I am not 100% sure on this. Its just what I believe based on what Ive picked up.