This should work for any device on any ROM
I have not tested the WVGA version since I don't have a Desire or Nexus (or something like that) so please tell me if it fits with the screen
Help me spread this awsome bootscreen: Post it on the theme section for your device!
1. Boot your phone into recovery mode by holding the "home-" and "power" button
2. Go to your backup menu and select "NANDROID BACKUP"
3. Flash the .zip file that fits your resolution
4. (If anything goes wrong) Go to your backup menu and select "NANDROID RESTORE"
If you would ever like to restore the original boot screen flash your ROM .zip on top of this but remember if you've made other changes to your ROM they will disappear (themes etc.)
HVGA: for devices like HTC Hero with the screen resolution 320 x 480
WVGA: for devices like Nexus One with the screen resolution 480 x 800