The functions inside FCP do not need the OS support. Apple can install private frameworks, and they do it already, for their own applications. So i think they will support SL.I doubt Apple will ship a new version of FCP before they ship lion, there are simply no real video editor APIs in Snow Leopard that are capable of 64 bit, QT Kit is a joke.
HOWEVER, according to the developer page for Lion there will be a brand new A/V API in Lion that will be 64 bit and FCP will most likely be written in that.
I guess they could back port the entire API to Snow Leopard, but I wouldn't count on it.
AV Foundation brings back QT7-features to QTX. Apple uses AV Foundation in the new QTX-player of Lion.