As both apps can now open the same Library, why have two? Just merge them with Aperture.
Other than that: open the iPhoto Library with Aperture. Select the projects you want to move and export them as an Aperture Library (it's an option in the Export menu) and then switch to the Aperture Library and import the one you exported, if you follow
What versions of iPhoto and apperture do you have? If you have Aperture and iPhoto versions that have the unified library format (Aperture 3.3: Using a unified photo library with iPhoto and Aperture), you can export your projects from iPhoto as library and import them into Aperture as library.
- Open your iPhoto Library in Aperture (Launch iPhoto and use the command "File > Open Library in Aperture")
- Select the projects and albums you want to use in Aperture in the Library Inspector.
- Use "File > Export > Items as new library"
- Then use "File > Switch to Library" to open again your main Aperture library and use "File > Import > Library" to import the new exported partial iPhoto Library, to be able to use it in your curent Aperture library.
You may need to update your iPhoto version to the latest version and to upgrade the iPhoto library by opening it once in iPhoto, to be able to do this.
Terence, could you please help with this thread, if you have time to look into it? iPhoto mistakes an iPhoto06 library for an Aperture library ...