I typed the answer below without clearly reading your question. It might be useful to others. What I think you're looking for doesn't exist, but this appears to be close - phoshare.
I know that out of the box the iPhoto library can be opened in Aperture.* That does give you some flexablity as you can switch between the two apps and the organization and metadata is kept intact.
The other thing to consider is that the iPhoto library is really just a collection of folders and and databases for reference. If the software that support is were to disappear tomorrow, you'd still be able to access your source photos.
If you navigate to your ~/Pictures folder and right (or control+) click on your iPhoto Library you can view its contents. Select "Show Package Contents" and you'll see the innards of your Library.
There's a folder called Masters, which is where all of your unedited photos are stored - in a hierarchy of folders by date.
In theory, you could extract and then import that into any myriad of tools down the road. You would obviously lose lots of reference data and organization, but you'd still have your files to do as you see fit.
*As of Aperture 3.3.2 and iPhoto 9.3.2