OK the 1 & 2 keys do stuff in Jelly too. And yes, you can write your own visualizers using Quartz Composer, I just tried modifying Jelly (which lives in System/Library/Compositions/ and saved it to /Library/Compositions and it shows up & works in itunes.
So it looks like Screensavers, Visualizers, and probably iChat & photo booth effects are all quartz compositions. But the compositions themselves vary depending on what they are used for, so Jelly would indeed need some work to function as a screensaver. Quartz Composer is fun but gets complex in a hurry so this stuff isnt going to be for everyone, the majority will be relying on others to make quartz compositions and share them. I think the doors are probably open for the R2D2 effect or similar to be put back into Leopard