The day has finally come. My upgrade protection has finally kicked in and I have managed to install Adobe Design Premium and Production Premium CS4 on my work machine (Mac Pro). That means I FINALLY have native AVCHD support in Premiere, making the cumbersome task of importing AVCHD video from my HD camera's SDHC card through a Windows VM and then copying it OS X for import into a video sequence non-existent. Praise Jesus. Really. Praise Him.
Interesting note on that though. Premiere CS4 does not import that AVCHD video footage the same way it would DV cam footage. And that ends up meaning that after I place the clips in my sequence timeline, I have to render them out to even get them to play. Which is fine. But it would have made more sense to just import them that way at the beginning.
Also, you should uninstall CS3 first, if you're on a Mac, backing up your custom brushes and whatnot. THEN install CS4. It will not automagically upgrade and then you are left with two versions of every program which is a lot of hard drive space. And if you were thinking of uninstalling CS3 after installing CS4, think again. It won't work. You'll break Photoshop (at the very least) and have to reinstall it anyway.
If you're on a PC, as far as I can tell the upgrade is automatic--though it takes a hell of a lot longer to install than it does on a decently spec'ed Mac. Just warning you. It was about 30 minutes to install Design Premium on my Mac and I'm going on one hour on just the first disc on a hoss of a PC running Windows XP SP3.
Overall, I am so far happy with CS4 - I think the color profiles were updated in Photoshop... and that's good.