Coverage and Network Compatibility
The main 3G network of WIND Mobile is AWS (1700/2100) and all reports are that the iPhone 5 will not include support for this. WIND's expansion of coverage into HSPA+ is, however, reason to cheer because all models of the iPhone 5 appear to be supporting that flavour of 4G.
What kind of coverage is possible on HSPA+ with WIND Mobile right now? It's actually not bad if you live in the Toronto area, take a look at a map here. This would appear that WIND can support the iPhone 5.
The bane of the entire industry, companies lock their phones to keep you from taking them to other carriers. You'll need to get the iPhone 5 unlocked. Thankfully, this is possible directly from Apple for a hefty price premium (currently starts at $699 on the apple.ca site).
Nano SIM Card Sales
The final piece of the puzzle is the SIM card that makes your phone work. This will, unfortunately, require WIND's active involvement by ordering and supporting this new SIM card technology. With T-Mobile in the US planning to offer nano-SIM cards to those with unlocked iPhone 5's, the question becomes whether WIND Mobile will do the same for their customers. I don't know of a hack that gets another nano SIM working on WIND, yet.
For WIND, supporting this phone is a win-win for them. Customers are happy using the phone they want, and WIND gets to keep customers that may have otherwise run off to an incumbent for the support. If I were in upper-level management or marketing at WIND Mobile in Canada, I'd get on top of Nano-SIM card support and make your customers happy. I'll update you as I continue to investigate this.
If you want to see this support happen, let WIND Mobile know everywhere they are: Online Contact - Twitter - Facebook