I expect it depends on the airlines; this hasn't happened to me often, but when it did, the airlines usually did call; not always in as timely a fashion as I would have liked though. I would probably use something like expedia or the airline web site to check on the flight from time to time, and then 2 or 3 weeks before, call (if you haven't heard anything).
Frontier just cancelled both the flights that we had in December: Denver to Orlando and then return. I noticed it before they had a chance to call (expedia notified me that there was a change in our itinerary), so I called Frontier. They had rebooked us on a different outbound flight earlier in the day (no problem); the return flight they booked us on returning a day earlier, since all flights on the day we had were cancelled - I asked to be booked on a return flight a day later, rather than come back a day early - no problem.
I think the key things are, if this happens to you, try to be calm and if possible, try to be flexible.
I know the person I talked to at Frontier was very happy that I wasn't angry and was willing to be flexible.