I'm working in Premiere and there's some stuff I'd like to do in AE. Can I import a whole project? What can go wrong?
Or is that a really really dumb question...?
I use PP CS3 and AE CS4
Yes, you can import a Premiere Pro project into After Effects.
Under the hood, this works the same way as if you were to copy and paste items from Premiere Pro to After Effects, so the limitations are the same as for copying and pasting. After Effects and Premiere Pro don't have all of the same features, so some aspects of your Premiere Pro project will be ignored or converted. There are tables that describe what gets ignored and converted in the "Copy between After Effects and Premiere Pro" section of After Effects Help.