I decided that I was happy to migrate en masse images with no adjustments. Fortunately, Aperture makes this easy. I selected one of my folders and chose Migrate Folder from the File menu. This gives you two sets of options. The first option is whether to directly migrate versions, or to create new versions using the RAW 2.0 decoder. The second option is whether to migrate all images, only images with adjustments or only images without adjustments. I chose to directly migrate only those images with no adjustments.
Note: this is insanely time consuming and memory-intensive. Aperture is migrating 800 images for me right now and it's eating 1.2GB of RAM and 320% of my 4-core 3GHz Mac Pro. I guess this entire process might take some time!
The second phase is what do do about all those tweaked images and, indeed, how to even find them. Turns out that finding them is easy. Aperture 2 allows you to search for versions by the adjustments applied to them:
So, there I have a smart album of all my to-be-migrated images. I plan to start with the best ones and work downwards from there. That way, if I get fed up with doing it, at least the best images will be done :-)