I'm currently thinking that for security, PMs will not be automatically transferred over. Certainly, there will be a way to download your old PMs for at least a year after the transition, and maybe there will be a way to import these PMs into the new forum if you want. (I already have code written for exporting your PMs as an mbox file which you can import into eg. Thunderbird, though I haven't gotten around to creating all of the interface stuff for it yet.)