Ars Technica has a great article on something new developing at Yahoo.
From the sound of it, Yahoo Mobile Developer Platform may be a lot like what many are expecting from Apple's iPhone SDK. Developers will be able to write a single mini-application that will be made available through Yahoo's Widget Gallery, where users can cherry-pick their favorite widgets to use on their phones. There are already a handful of widgets available from big names like MySpace, MTV, and eBay, and Yahoo said that a full SDK with guidelines would be available "soon."
Yahoo! Mobile Developer Platform
Yahoo! Mobile Developer Home
Yahoo! Mobile Developer Blog
Hopefully this will encourage Microsoft to release SideShow for Windows Mobile, before someone else does it first.