Justin Williams and Nicolas Haunold, also known as Jaywalker998 and westbaer respectively over Twitter and the members of Chronic Dev Team, have just posted a video on Youtube showing a jailbroken iPhone booting with custom bootlogo.
This new feature is coming in the next Greenpois0n RC5 update which will support untethered jailbreak on all idevices up to iOS 4.2.1.
According to Jaywalker998:
The current plan is for this animation to be available for your boot animation via Cydia. Additionally you will have source to make your own
Not fully ready for release, but getting there. For those who can’t tell, the second image is the GP Skull :D
Here is the actual video: