While I am *very angry* at Apple (what a shockingly bad release, were you all sleeping or something?) for not testing a major O.S before release, I have found a work-around for anyone who (like me) is knackered if you can't capture.
Start iMovie, wait for it to spawn its own capture window when you switch on your player/camera.
Close this window and then:
In (FCP) capture settings, select non-controllable devices.
No idea if this works for others of you, but I'm trying to share what I found.
There must be a way to fix this by Apple, if I can hack it by mistake a patch must be possible.
*I am disgusted and disappointed in you Apple for not testing SL properly*, we all expect so much more of you, but FCP is much better than anything I can use on a Windows machine.