Yup. There's a bug in iPhoto 9.4. The problem has been reported to Apple, and they are reportedly working on a fix.
You can live with it for a little while until 9.4.1 comes out, or if you have a Time Machine backup you can revert to iPhoto 9.3
There's also a way to edit your albums and slideshows in order to get it to work again (it involves looking of "&" characters in album names or slideshow music) - but it doesn't work for everyone and it's probably best just to wait. See this conversation if you want to try to fix it now.