I agree with this in principle, but in our Web 2.0 world, "beta" has become almost meaningless half the time. Gmail has only come out of beta last year, hasn't it?
Perpetual public beta is a problem, but there's a huge difference between a public beta from a huge corporation and a small private beta from a startup. I think it's pretty easy to judge based on company size, service momentum, number of competitors, etc. if a service is likely to last.
Surely you already knew this principle.