Microsoft announced today via its Windows Phone Developer Blog that it is now accepting submissions for mobile apps that run on the soon-to-be-released Windows Phone 7.5 "Mango" mobile OS. The company says that now is the time for developers to submit apps that use new Windows Phone 7.5 app features like fast app switching, background audio, multiple and double sided Live Tiles and better Search integration.

While new phones based on Windows Phone 7.5 are still some ways away--as is the free Windows Phone 7.5 update for existing phones--Microsoft's Todd Brix notes that those running pre-release versions of the OS will soon start seeing new 7.5-specific apps appear.

"It's time to bring those [Windows Phone 7.5] apps to market!" he writes. "The next generation of Windows Phone apps will soon be hitting the market. Now is the time submit the must have apps for the new wave of [Windows Phone 7.5] phones."

Additionally, Microsoft announced the delivery of the Release Candidate (RC) version of its Windows Phone SDK 7.1. (Remember, Microsoft confusingly calls the OS version 7.5 even though the underlying version number is 7.1.)  This comes with a "Go Live" license, meaning that developers can use this pre-release code to ship new apps for Windows Phone 7.5, and the Marketplace Test Kit, which allows developers to test apps for certification using the same tests that Microsoft's Marketplace team runs.

Microsoft provided me with the following list of action items for developers interested in Windows Phone 7.5 development:

The WP SDK 7.1 RC (Release Candidate) tools, available in English and Japanese, include the 'Go Live' license that lets developers publish their apps to Windows Phone Marketplace.

Developers should submit their apps now to become a part of the must-have app selection in time for the new wave of Mango phones coming this Fall.

Game developers can now submit games to the 19 new Windows Phone 7.5 markets announced in in July, including: Brazil, Chile, Colombia, Czech Republic, Denmark, Finland, Greece, Hungary, India, Japan, Netherlands, Norway, Poland, Portugal, Russia, South Africa, South Korea, Sweden and Taiwan

The WP SDK 7.1 RC includes the completed Marketplace Test Kit that allows developers to test their apps for certification requirements before submitting to Marketplace, ultimately speeding up the app certification process and the time it takes an app to hit Marketplace.

Developers can easily cross submit games and ratings information to multiple markets via App Hub.

For those developers interested in making money with in-app advertising, the Microsoft Advertising SDK for Windows Phone is now installed as part of the WP SDK 7.1 RC tools, making it easier for devs to get started monetizing their apps.