I've read a few reports from others that migrant waves can take until the second year or later now. That's pretty interesting since Toady didn't mention such a change specifically, but he did mention working on the population cap bug. Maybe it's related to that somehow? In any case I wouldn't jump to call it a bug unless you get no migrants ever.
Toady's quote on his popcap fix in the bugtracker:
I've tried to make this a little more useful, and it should control migrant waves now, to the point of preventing families that are too large from coming and also stopping generated families from bringing children and so on. Monarch arrival can still break the cap, but that should be an isolated event. I also added another cap that will stop fort births from violating the cap, and set the defaults to 200/220.
Popcap is also controlled by the two init settings, one the old one, and the other the strict one.
But no clue how that affects migrants. Maybe they are like the Liaison now? That migrant waves have to cme fro somewhere, and sorta get sidetracked?
That'll make dead-civ forts really hard(or ideal, for those generational fort-nuts).
Edit: apparantly people have been overwriting their init_d files, not realising that the strict pop-cap has been added. This results in the strict pop-cap being 0, and thus no imigrants.