This very same question has been asked at the info session in Sao Paulo. The presenter said this:
"After an immigrant becomes a permanent resident, it's the province's job to keep him in the province that nominated you."
He even joked about been accepted as Microsoft's CEO at a different place.
What he meant is, the province has to provide us, wannabe nominees, with the "tools" to be able to settle there and grow roots. Yes, they ask you for a commitment to stay within their province, but the canadian constitution guarantees you, as a permanent resident, the freedom to relocate to anywhere you see fit in Canada.
Read this, fro attorney Colin R. Singer
There isn't such a thing as to a minimum period of time that one has to stay at any give province. During your landing (remember you're still not a permanent resident at this point) you have to prove to the immigration officer that you are, indeed, going to the province that nominated you (for example, you do your landing procedures in Toronto, but you have a flight booked to Moncton, an address to hotel where you made reservations there, and etc). After you clear through the landing procedures, you'll be given PR status.
Then, the aforementioned information will be valid to you. You can move anytime you want. Just don't be an a$$hole and go straight out to Toronto, Vancouver or whatever. At least try to start and make a living at the province that mentioned you. After all, you did accept their nomination right? You pursued it! At least try to make it work. If it doesn't, and you ran out of alternatives there, then go somewhere else.
Simple as that...