Just like every other Android phone, the first part of the boot sequence is hard-coded into the ROM. The "HTC" or "M" or whatever depending on the brand.
So you'll have to wait for somebody to cook new ROMs to get rid of the "OMFGWTFBBQ that's too bright and too white" AT&T World Phone.
The Vibrant has a great startup, btw, just says "Vibrant" in the middle in white, and Galaxy S at the bottom, mostly black screen and it's tasteful and has impact. The animated portion...not so much, it's arguably worse than the Captivate (at least once the AT&T "great bars of fire!" bit is over.)
The animated part, that's pretty much done the same way the shutdown sequence is (a series of .png stills linked together, and the order of the playback controlled by a .xml file). Unlike the "S" (or Vibrant "Goodbye"), which is an optional non-standard Android behavior, the method in which the boot animations is packaged is important.
I imagine you'll start seeing new boot animations once we've got a working nandroid backup system.