Apparently the latest version of the Adobe Flash plugin is not totally compatible with Leopard. From what I can tell, you can play Flash in your browser, but some functions won’t work… specifically Flash uploading.
For instance, the photo-sharing site Zooomr.com uses a Flash tool to upload photos. They’ve posted a message on their uploads page that states Leopard users will NOT be able to upload photos to their account until Adobe fixes the Flash plugin.
Luckily, I can’t think of any online tools I use that use a Flash upload tool, but it’s something people considering upgrading to Leopard should be aware of.