on our wishlist but currently not a requirement for the 1.0. slotted for second release
jimm: I would expect the level of effort for metro to be fairly light, BUT, I would recommend waiting until bug 803719 lands, as it will improve the reflow-on-zoom feature quite significantly and might involve some front-end stuff that would be needed to understand the difficulty of implementing this feature on metro. That said, provided that you don't have to re-implement the backend platform stuff (I don't know how metro works), it should be pretty low effort to get this working on metro.
I don't think this makes sense for Windows 8 Metro, which has a minimum screen size of 1024x768. Resolving wontfix for now. We can revisit this if we extend the Metro UI to other form factors. It's also easy to enable already using hidden prefs (via about:config or an add-on).
Windows Blue, due out later this year, takes Windows 8 down to 7" tablets. I don't know if that will change their resolution requirements or their scaling system or their guidelines. I'm OK with this bug going away and if we decide to do this, I will create a user story for it and we can go from there.