Strange, I use my old iPhoto library with Aperture since they are the same format. I can open the library with either app. The default app for an iPhoto is iPhoto but Aperture should be able to open it. Was iPhoto running when you tried to open the library with Aperture?
I'm not sure. I'll try this method again w/ iPhoto closed. I'll report back.
----------
I think that I see what the problem is. When I go into Aperture (w/ iPhoto closed), and I look in the File menu, the only "Open" option I have is "Open Library in iPhoto." That's why it opened in iPhoto, apparently. I don't see any option to just "Open" a library.