The way I understand it, the new AC pilots contract dictates that half the F28's have to be gone by 2003, with the rest by 2005. There is a restriction limiting AC regionals to 39 RJ's which have to be 55 seats or less (maximum capacity, they can't buy larger aircraft and put fewer seats in). Also, before the CRJ's can be transferred to the regionals from AC mainline, they have to get replacements in the mainline fleet.
Effectively this means that CRJ's are most likely going to be the F28 replacements for the regionals, probably the 25 AC already has plus a few orders to bring it up to the 39 aircraft limit (or maybe more, the limit increases as AC mainline fleet grows).
So to answer the question, no the phase-out hasn't yet begun, but plans will have to be announced fairly soon...
Also, as to 732/DC9 phase-out, I suspect that the additional 319/320/321 aircraft will go PART of the way to either replace 732/DC9 aircraft on some routes, or maybe 320/321's will replace 319/320's on some routes to free them up to fly 732/DC9 routes. In the end, IMHO, I think we'll see the longer haul 732 routes (ie- YEG-YUL) flown by 319's, while probably another fleet type (ie- possibly 717's) for some of the shorter runs.
But that's just my opinion, I'm still waiting for phase II orders to come out.