Windows Phone 7.5 "Mango" is tracking for availability this fall on existing and new handsets alike, and to prepare for the launch, Microsoft has offered developers a pre-release cut of its Mango development tools for most of the summer. Now, Redmond is taking it to the next phase by opening the door to Mango's Marketplace, meaning developers are welcome to submit Mango-compatible apps that take advantage of the new build's many new features: fast app switching, improved Live Tiles, background audio, deeper camera support, and more. Additionally, apps can now be submitted to Marketplace locales that are being supported in Mango for the first time, including Brazil, Finland (a boon for Nokia, no doubt), Russia, South Korea, Taiwan, and some 14 others. We don't know exactly when the first Mango devices will launch at retail, but Microsoft says that devs should submit their wares right now if they want to be included in the first round of apps, so it's likely coming up very quickly.
Source: Windows Phone Developer Blog