I've had Aperture 3 for a couple weeks now, and I've been reading some great Aperture 3 blogs (such as http://photo.rwboyer.com/) and a lot of frustrated forum posts about the pain people are having converting their libraries from Aperture 2 to Aperture 3.
After some painful experimentation I'm happy to report that I've found a simple solution that has be relatively pain free merging about 10 managed libraries with about 100,000 photos into one referenced library.
The problems: Lots of people have been reporting problems trying to upgrade libraries to Aperture 3. Migrations run overnight and then crash right at the end... etc, etc, etc... Lots of discussion about repairing all your libraries in Aperture 2 before you start upgrading... etc, etc, etc...
The solution: DO NOT TRY TO UPGRADE YOUR EXISTING LIBRARIES!!! Instead create a new Aperture 3 library(s) and IMPORT your old library(s) into it. The key point here is that Aperture 3 can import from earlier library formats without upgrading them. So far this process has been almost problem free for me.
My issues: The only issues I had with this process were with a couple of libraries that I had copied to a Linux NAS device. Apparently Aperture libraries do NOT like being stored on non-MacOS formatted drives (I knew this about external FAT USB drives, but I had mistakenly assumed that *nix XFS would be ok). Fortunately, I had alternate backups of these libaries on Mac formatted external drives (redundancy is your friend :-).
