Gad, I thought I could figure this out on my own, but no.
I have text moving L->R along a path. At the end of the path I want
the characters to "fall off". If I animate the position past the
end of the path, the text just continues on it's last trajectory. I'm currently using CS4.
You could break the handles and change the outgoing bezier handle on the end point so the departing curve points straight down, or just make a longer path and change your L->R animation to be a portion of your mask path.