This matters more than ever now that Google has started phasing out support for Exchange ActiveSync for their e-mail, contacts and calendar products.
@fruux: Understood. Thanks for considering it.
Even with the upcoming WP8, I'm not confident of Windows Phone implementing CalDAV or CardDAV, so the next best thing would be ActiveSync support. There is at least one open-source solution out there, being Z-Push (http://z-push.sourceforge.net/soswp/), which is being implemented by some other services that feature ActiveSync services. I think this can be a real killer function for multi-platform users such as myself, and entice similar users to use Fruux as a service.