Last evening, the GP Animate Disabler tweak appeared in Cydia, which disables the animated boot logo installed by greenpois0n RC5 after jailbreaking. Once installed, the tweak directly does its "work" and removes the greenpois0n boot logo, replacing it with the classic Apple logo that you see before after every restart. For now there are no methods to make custom animated boot logos, but in the near future some applications should appear in Cydia either with ready-made animated boot logos or that allow us to make our own, or some methods will be available through that we could make logos from the computer.
GP Animate Disabler can be installed for free from Cydia in the repo http://repo.insanelyi.com