There can't be more than one person in a million who cares about this.
This post is for you. Please comment so I know I'm not alone. (Just joking, I know I'm alone.)
I've been curious about how metadata (title, comment, etc) passes between Aperture 3 and iPhoto 8.1.2 
I ran an experiment today to find out. I started with a RAW image. I exported a JPEG version to the desktop then dropped it into iPhoto. I also, for the heck of it, used iPhoto's Aperture browser and dropped an image in that way. 
Here's what I found (see  below for a note on the table).
- n/d means not displayed
- e- means it can be seen in the EXIF details on Picasa Web album
|Aperture Attribute Name
||Caption, e-object name
|Image Location (text)
|Image Location using Places
So if you, for some strange reason , edit in Aperture but store in iPhoto, don't bother rating photos. You can, however, use the following attributes and see useful information in iPhoto 8:
- aperture.Title -> iPhoto.title
- aperture.Caption -> iPhoto.description
- aperture.Keyword -> iPhoto.keyword
- aperture.Version Name -> file name if specified during export
- aperture.Places -> not rendering for me in iPhoto 8, but it's stored correctly and Picasa Web Albums can use it.
When exporting from iPhoto to Picasa only iPhoto.title and iPhoto.keyword are used.
Based on this experiment, I crated a custom Aperture metadata set that included Title, Caption and Keywords. I also customized my Grid View - Expanded metadata (cmd-J) to include Title, Caption, Keywords and Version Name.
Update 9/7/11: It appears that the Aperture Project Name is written to JPEG EXIF during export and read by iPhoto during import. Most surprising.
 This really surprised me. In the past this metadata had been preserved. I wonder if an Aperture update made it incompatible with my older version of iPhoto. Although iPhoto 8 couldn't read the location metadata, it was in the EXIF header because Picasa could read it.
 Something odd happened here. I'd assigned a Caption on Import and that's what showed up in iPhoto. I suspect it was IPTC metada from the RAW image.
 This can become part of the file name on export from Aperture. The iPhoto.title attribute can be set equal to the file name by batch update. So there's a way to pass this to iPhoto if desired.
 Exporting from Google to Picasa Web Albums using Google export
 This isn't something you'd normally do. It just saves a @500K JPEG Aperture uses as a preview images. Still, it's interesting to see what happens with the metadata.
 When I tried to create this table I again mourned the passing of FrontPage, Windows Live Writer (all but gone) and the great wysiwyg editors of old. Neither MarsEdit (this tool) nor iWeb do tables. So I downloaded SeaMonkey (88MB - once that was a lot). Since I remembered Netscape Composer I had a major flashback with fascinating visuals.
 I'm stuck in iPhoto until Apple changes Aperture's iPhoto import to include more metadata. Also, I don't trust RAW for archival storage. I save JPEG and discard RAW.
 I haven't updated to iPhoto 9, the dead fish smell has been offputting.