Update: In case you didn't figure it out for yourself, Arnoud from Tweakers.net wrote in with confirmation that the no-VOIP rule is in place at the request of the providers.
[Warning: PDF read link]
- Applications that are or distribute alternate marketplaces for content types (applications, games, themes etc.) that are sold or otherwise distributed through Windows® Marketplace for Mobile.
- Applications that link to, incent users to download, or otherwise promote alternate marketplaces for content types that are sold or otherwise distributed through Windows Marketplace for Mobile.
- Applications that promote or link users to a website, or contain functionality within the application itself, which encourages or requires the user to purchase or pay to upgrade the application outside of Windows® Marketplace for Mobile.
- Applications that enable VoIP (Voice over IP) services over a mobile operator network.
- Applications that sell, link to, or otherwise promote mobile voice plans.
- Applications that display advertising that does not meet the Microsoft Advertising Creative Acceptance Policy Guide http://advertising.microsoft.com/creative-specs.
- Applications that replace, remove or modify the default dialer, SMS, or MMS interface.
- Applications that change the default browser, search client, or media player on the device.
- Applications with an OTA (over the air) download >10 MB.
- Applications that run code outside Microsoft runtimes (native, managed, and widgets)
- Applications that publish a user's location information to any other person without first having received the user's express permission (opt-in) to do so, and that do not provide the user a means of opting out of having their location information published.
- Applications that publish a user's data from their mobile device to any other person without first having received the user's express permission (opt-in) to do so, and that do not provide the user a means of opting out of having their data published. A "user's data" includes, without limit, contacts, photos, SMS or other text communication, browsing history, location information, and other data either stored on the mobile device or stored in the "cloud" but accessible from the mobile device