Among the items that were talked about on Monday during the Ubuntu 13.04 Developer Summit in Copenhagen were how the SDK is to be integrated with Ubuntu, language/tool-kit support, form-factor support, performance, availability of documentation, the stability/maturity/support-life of an Ubuntu SDK, application sandboxing, and abstraction of the actual implementation.
The API for the Ubuntu SDK is expected to be stable and maintained for some time and will also incorporate support for all form-factors of devices where Ubuntu is supported from desktops to mobile devices and the eventual Ubuntu TV.
The SDK is expected to be made of a set of APIs, a root file-system / container, the necessary build tools, a back-end ecosystem, and documentation.
Areas that are likely to be covered by this Ubuntu SDK include GUI, audio, video, networking, file I/O, localization, accessibility, internationalization support, test coverage, data syncing, online accounts / credentials, background processes / service, user configuration and settings, tools/IDE/compilers/packaging, meta-data tracking, indicator support, notification support, and application documentation.
Expect an Ubuntu SDK to emerge at some point in the not too distant future, but it's not being planned for the Ubuntu 13.04 release in April.