Plymouth will not be arriving in time for Jaunty, but it is currently planned to replace the Usplash in Karmic Koala (Ubuntu 9.10). However Plymouth is now available for testing in Jaunty though PPA:
You'll need kernel 2.6.29 which you can get from here:
There should be a PPA for the kernel you can add to your software sources soon, but for now this should work for testing purposes. Plymouth seems to be more CPU consuming than Usplash however, the boot time is unchanged.
Some people are hoping the Ubuntu Usplash Smooth will make it into Jaunty, but there hasn't been any sources confirming that. With Jaunty in feature-freeze, this now seems highly unlikely.