When using Flash with Floatbox, is there a warning displayed if the user does not have a late enough version of Flash to view the video? I thought I saw information on this in the instructions or somewhere else, but I can't find it.

No, there's no flash version checking done. Perhaps there should be. I'll look into it as a possible future addition. In the meantime, if you need version checking you'll need to put your flash on a self-contained page with the config and features you want and then load that page as an iframe in floatbox. Works fine, is a little more coding than direct loading, but gives you more control.