It's achieveable, but somewhat of a hack, and the best you'll get is something close to your event structure, but not albums.
I'd really recommend lêonie's approach above.
However, if you really want to play...:
There are two main problems to solve.
1) The files are inside the iPhoto library bundle which is not selectable from the import panel.
2) You'll have a combination of master, non-destructive edits, and previews. I no longer have an 8.1 library to check so I can't decribe the structure). You'll need to decide if you want to bring in originals and edits (where available).
You'll need to have a look around the iPhoto library structure; navigate to it in finder, right-click the library and choose 'Show package contents'.
Explore this until you are happy you know where the images are stored (orginals and edited copies), these are the images you can access directly.
To make them accessible for import in Aperture, drag any folders your are interested in (eg: originals, masters, previews, etc) to your finder favourites sidebar. This doesn't move them anywhere, it just gives you shortcuts to those folders that reside inside the iPhoto library bundle.
In Aperture, use 'Import->Folders as Projects' from the file menu. Here you can specify that the images stay in their original location (imported as referenced) and you can use the shortcut on your finder sidebar (created above) to navigate into the iPhoto library to pick up the images.
You can try the different options for the stucture of the import ('Folders and Projects' or 'Projects and Albums'). 'Folders and Projects' will probably work best as it will give something approximating your import history which may match your event structure, but the project names will likely not match the event names.
Once you've got them in Aperture, youll probably need to tidy up the structure, but in Aperture you can move and change projects as much as you want withouth affecting anything in the old iPhoto Library. The things that can impact the iPhoto Library are deleting or relocating any of the masters.
As I say, it's a bit of a hack, and in the long run you will probably be better off by just importing the library and relocating the masters. This will keep your event and album structure and be much cleaner, but it will give you an additional copy of the images, which I suspect is what you are trying to avoid.
Also, if you get future problems with your images, you'll have to explain all the above or people may have trouble helping you!
Andy