After years of false claims of iPhoto to Aperture migration support, and four months after I realized iPhoto was going down the wrong road for me, and three months after I began my extremely painful iPhoto to Aperture migration, Apple has released iPhoto Pro (aka Aperture 3.3).
Imagine my joy.
The only saving grace is that I might have saved a few people by advising them to wait for Aperture 4, which has now come in the form of iPhoto Pro 1.0/Aperture 3.3.
According to Apple's marketing claims, the latest (Lion-only) versions of Aperture and iPhoto share a single database model. So, in theory, both apps can work on the database.
I doubt it works as advertised, but it has to be an improvement on my experience! I'd strongly advise waiting until September before doing a major iPhoto to Aperture migration. Now that you know the end is in sight the wait should be tolerable.
Personally, I'm looking forward to, at the least, using iPhoto's Picasa uploader and iPhoto's superior UI for Event/Project and many common image management operations.
Assuming the inevitable bugs get sorted out, this is an extraordinary conclusion to what must have been a formidable software effort. At one point Apple had two completely incompatible photo management products. One was a natural Mac app with an elegant UI and some infuriating limitations. The other looked like a port from NeXTStep . They had almost nothing in common.
Slowly, painfully, Apple turned these two disparate products into iPhoto and iPhoto Pro. To do that they had to reconcile very different functional models and data models. It would have been a very hard, very long, glamor-free slog. I hope the team was at least paid well.
Now, if you'll excuse me, I'm going to go cry in my Scotch.
 I was never able to learn where the heck Aperture came from. It's neither Windows nor Mac.
Update: I remembered my underused MacBook Air runs Lion, so I tested the new Aperture and iPhoto together. Unsurprisingly, they failed my first test.
First -- the good news. Annotations on an iPhoto Event are now visible in Aperture.
The bad news -- Album annotations (descriptions) are still not viewable or editable in Aperture.
Maybe in another year or two?