I didn't even have dust on the box yet, but Aperture 3 has scored an update within a few days of its release. The update, according to Apple, "...addresses an issue affecting the playback of video clips used in Aperture 3 slideshows on Snow Leopard."
The update is 65.2 megs to download. If it hasn't popped up for you yet, you can force it from the Apple Menu by selecting 'Software Update.'
I can't tell you it's a great fix because I haven't tried any video in my slideshows yet, but Apple suggests all users get the update.
I other words, be there, or be square.
[Thanks to Andrew for the tip]