The file format of the Library is now the same. It is now no longer correct (after the updates) to say "I have an iPhoto Library" or "I have an Aperture Library". Updated Libraries can be opened in either program. (There are limitations -- none major -- see below.)
It does not matter (afaik) which program you use to import files into your Library. The Library can be opened with either iPhoto or with Aperture. All imported files will show in the Library.
Here are a couple of articles from the always-useful Aperture Support page:
Does this mean there can now be "one library" that is shared between both?
Yes, if you have Aperture 3.3.x and iPhoto 9.3.x, see: iPhoto '11: Use iPhoto with Aperture
I now have a serarate libray for Aperature and a separate libray for Iphoto. Can these libraries be combined into a common library? If so, how do I combine them?
Yes, they can be combined into one library. After upgrading both applications to the latest version, you need to open your iPhoto library at least once in iPhoto 9.3.x to upgrade the library, and your Aperture library in Aperture 3.3.x.
Then import your iPhoto library into the Aperture library with "File > Import > Library". You will need plenty of free disk spcae on the volume, were the merged library will rside.
If I can have a common library, would I just import all camera photos into that "unified libray" and them be able to access it for editing from either program?
Yes, you can use the merged library from either application, and edits will be visible in either program. There are some limitions with respect to print products, slideshows, and file types, see: http://support.apple.com/kb/HT5249
Kirby, I see, we have been cross-posting!
when you are importing the iPhoto Library into your Aperture Library, you will be prompted if you want to merge or to add the iPhoto events, that are duplicates, provided Aperture can recognize those images are duplicates. Then you will see this panel on import:
Select "Merge". Next you will be asked, which version to keep on case of duplicates - the version in the library you are importing to, or the imported one.
If you do not see the above panel, then Aperture cannot recognize the duplicate events. But it may be worth it to import those again, and than to delete the older copies, for now Aperture makes a much better job of importing. It can create "original-version" pairs and does no longer import "master" and "version" as separate images.
Alternately, you may want to remove the events, that you know you imported alredy from your iPhoto library before merging, after making a backup of both libraries.
The file format of the Library is now the same. It is now no longer correct (after the updates) to say "I have an iPhoto Library" or "I have an Aperture Library".
Widely true, Kirby, but to the system the distinction still matters. Here are two brandnew libraries, one created in iPhoto and one in Aperture at the same time; the Finder shows for them a different file extension, a different kind, and a different version. The difference will be noticable, when you want to sync your images to your mobile devices from iTunes or when you want to access the images using the Media Browser. Then you need to know the application that created the library and pick the correct app to access the documents - one of the "blessings" of the new application centered document retrieval in Mountain Lion (probably).
A bit wrinkly for beginners, I imagine.
For most of us, I assume. Just look at the "I cannot sync my iPhone" threads. I still have no clear picture of how to control which library will be synced by which app, I resorted to a work-around and have the list my of my projects in each library begin with a dummy title project, called exactly the same as the library, Then I can see in the Media Browser which library is presented by the Media Browser