Yahoo has officially opened their mobile Widget platform to developers, including the ability to test and submit widgets. The announcement is on the Yahoo Mobile Developer Platform Blog.
To get started, download the SDK from the Yahoo! Mobile Developer Home. It contains a Developer's Guide documenting the XML based Blueprint language with sample code, examples and tips. The SDK also has a sample PHP script which dynamically generates Blueprint code for a Twitter Widget. For peer to peer support and tips there's a new Yahoo! Mobile Developers Group on Yahoo Groups which has active participation from Yahoo developer Ed Voas. one of the creators of Konfabulator, the original widget platform for MacOS. Konfabulator which was aquired by Yahoo a couple of years ago is the basis of Yahoo Desktop Widgets.
When you are ready to test your snippet or widget you can upload it from the Developer Home Page and test it on Yahoo's mobile portal. For testing Yahoo makes your widget available to you when you log into Yahoo Mobile. When you have everything working smoothly you can submit you finished creation to the Yahoo Widget Gallery and to the world.