iPhoto2 was slow, and very very slow when you got a lot of photos. Many others like me started to use iPhoto Library Manager to keep photos in several smaller iPhoto Libraries. With iPhoto4, Apple has fixed the worst performance problems, and I wanted to reunite all my photos. But how?
I put in one of the iPhoto CDs that I made with iPhoto2 and it showed up in iPhoto4 without a problem. On a whim, I selected all the albums on the CD and just dragged them to the bottom of the album list; I saw the green plus sign and dropped them. The import took some time, but to my amazement, *everything* was preserved! Albums, rolls, dates, originals, everything. Now I finally have all my photos in one place. I hope this works for others also.
You should, of course, make backups before trying this, but you did that before you installed iPhoto4, just like Apple told you to, right!?
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040127191712305