Wind Mobile's Home Zone coverage go up as far north west in Georgetown (Highway 7 and Trafalgar Road area) and just as far as Highway 410 and Mayfield Rd. in Brampton.
Judging by the future Home Zone coverage, the Wind Mobile Home Zone coverage will eventually reach towards Caledon tentatively in 2010. As you move outside of Wind Mobile's Home Zone coverage, you will roam through Rogers' coverage (if i'm not mistaken). Charges may apply.
I don't use Wind Mobile myself, but I hope this information will help. Cheers.