Terence
Thanks for your comment. Both my Snow Leopard desktops have iPhoto. 7.1.5 and QT player 10.0. The server always tells me that my Software is up to date, yet it seems from your comment they are not compatible?
And if I export those Video clips from iPhoto. 5 directly to a Snow Leopard folder (rather than transferring the whole iPhoto. Library), then QT Player 10.0 does open and run them!
Could it be that when iPhoto 7 updates the newly imported iPhoto 5 Library, it renders any included video clips unreadable to QT Player 10.0?