Here's a boot animation that I made in some free time I had. This was designed for the Droid X (if it matters).
It alternates between two states. The first one has the blue background with the little pixelated white stars and says "Booting NYANDROID...". The second state is just the poptart cat running.
For those of you who would like the annoying sound to be played, I made one with that before I realized how annoying it got every boot.
That one is here:
The less annoying one (no sound), is here:
In both cases, the bootanimation.zip gets pushed to /data/local (so you don't overwrite the stock animation). Do a backup of any other custom animation first (a rename is fine). To get the sound working, make sure you downloaded the sound version and push boot.mp3 to /system/media/audio/notifications on Gingerbread or /data/local or /system/media on Froyo.
I am running Gingerbread so I don't know if the location on Froyo is 100% correct for the sound.