Hi Kirby - there may be a more efficient way, but let me tell you what I'm doing as a LR / LR2 user who moved to Aperture 3:
You can create as many libraries as you want and then switch between them. As you likely also know, you can import files into the "library" but store them in place, i.e., where they are now.
So under portraits, I'm going and exporting the developed versions as JPEGs and putting those in a folder along the lines of LR Developed. That folder will sit with another with the masters that I've just imported in place under a single project.
What I'm finding is that I like Aperture 3 so much more, that I keep the developed JPEGs from LR for expediency but I'm reworking several in Aperture from the RAW file.
Much (all?) of the metadata comes over with the JPEGs I've noticed, but I haven't looked at the LR raw files.
Keeping those JPEGs with the RAWs isn't stellar because of course, they aren't linked so that's another reason why I'm reworking RAWs in some cases. And when I say reworking, one of the things that I like about Aperture 3 is that I like the way files look SOOOO (a little emphasis) from the start that I'm adjusting them much less than I did in LR2.
Check out www.maccreate.com
to see if there are discussions on this - there's an Aperture forum and some training (e-book - there's a large one there for $9.99, I believe it was) and video resources too (the Sara France one is fine after the first 15 minutes which are useless... perhaps I should care what inspired her to become a photographer, but I don't, not even a little bit and so more than 30 seconds on that is a bit much, but once she gets into the heart of things she does a fine job).