Viewing a slideshow in thumbnail view?

I have never tried to do as you describe while working in the slideshow mode, and I do not know if it is possible. I first create an album that contains the pictures I plan to work with. Then I work in the thumbnail view to rearrange the order of photos. I also take this opportunity to do any necessary editing. From there, I create the slideshow.

I hope this helps.

You can't. What you see if what you get. As Glo H suggested an album with the original set of photos could be arranged how you want and then make a new slideshow. But if you've created unique KB effects or transitions you'll have to redo all of them.

TIP: For insurance against the iPhoto database corruption that many users have experienced I recommend making a backup copy of the Library6.iPhoto (iPhoto.Library for iPhoto 5 and earlier) database file and keep it current. If problems crop up where iPhoto suddenly can't see any photos or thinks there are no photos in the library, replacing the working Library6.iPhoto file with the backup will often get the library back. By keeping it current I mean backup after each import and/or any serious editing or work on books, slideshows, calendars, cards, etc. That insures that if a problem pops up and you do need to replace the database file, you'll retain all those efforts. It doesn't take long to make the backup and it's good insurance.
I've created an Automator workflow application (requires Tiger or later), iPhoto dB File Backup, that will copy the selected Library6.iPhoto file from your iPhoto Library folder to the Pictures folder, replacing any previous version of it. It's compatible with iPhoto 6 and 7 libraries and Tiger and Leopard. iPhoto does not have to be closed to run the application, just idle. You can download it at Toad's Cellar. Be sure to read the Read Me pdf file.