I’ve lost several days to Aperture 3. In short, Apple should have done better. The Flickr export is sending photos at 1/2 resolution, Faces causes Aperture to hang or simply makes Aperture get stuck Processing… After some trial and error, this is what worked for me.
How to run Aperture 3 in 32-bit mode:
In order to use any plug-ins with Aperture 3 (currently), you will need to launch Aperture in 32-bit mode (in Snow Leopard).
To do this, go to Aperture, File > Get Info…, and select the “Open in 32-bit mode” checkbox.
This will be fixed as plug-in developers recompile their plugins.
(Note that using a plugin from 32-bit mode is supposed to cause Aperture 3 to quit and restart in 32-bit mode transparently—it will then restart in 64bit mode on next launch. YMMV)
Hopefully this works for you.