Importing Picasa Photos
By the way, importing Picasa photos takes a little planning. First, be sure to go through Picasa and save all your edits. This will create an Original subfolder that will eventually have to be ignored on import, or deleted once in iPhoto.
Use the iPhoto import feature, and all the Picasa folders will end up as events in iPhoto. If you have a folder structure in Picasa, it will be flattened on import.
Events in iPhoto are organized by the dates of the photos in each event. The best way to get started organizing your events in iPhoto is to first go through the imported events and get reasonable dates on all photos. There is a Batch change feature in iPhoto that is very helpful for this purpose. Once all photos have good dates on them, just start combining events and moving photos between events to get stuff the way you like it.
Any work you have done on Picasa albums will have to be redone in iPhoto. i Photo Keywords are a good alternative approach.