As far as I know, there are no plans to develop apps for Windows Phone, BlackBerry, or Symbian. If you need this feature, there's this mod available in the market: http://www.boonex.com/m/WAP_site_version_
It's not a native app, but any phone with a decent browser should work.
Originally Blackberry was in the lineup, they had promised a blackberry app, but later dropped this. This made blackberry fanatics such as myself very upset at the time, but looking at the way the market (world phone market) is going, I can't really say I blame them. iOS, Android, and with a bit of luck...Windows Mobile look to be the main leaders going forward. Symbian is a great idea, but it's execution is a bit of a joke, and Blackberry is lagging behind when they used to be a major market see more leader. All that's keeping blackberry afloat is their superior e-mail handling and business that relies on this. The way android and iOS work, it's just a matter of time before some vastly superior app for those phones is released that will allow for full PIM sync (and not the SAD excuse for this they have now). When that happens, blackberry will be dead.
Nokia slept with Microsoft and has never been the same, BlackBerry is a pain in the arse to develop for, and webOS is dead because HP has shown once again how incompetent they are.
Indeed, besides the big two (iOS and Android) the only other OS that has some chances to take off is Windows, but so far it's just a possibility. We're not going just by numbers... even our personal impressions give some clues - iOS is slick and usable; Android is flexible and usable; WinOS is fancy and not usable (slow, awful keyboard); BB is limited and Symbian is... well, pronounced dead.
It's not a native app, but any phone with a decent browser should work.