Just saw this over at the SharePoint Online Developer Resource Center:
Office 365 Developer Training Kit
Use this online or downloadable training course to help you get started building cloud-based communication and collaboration solutions for Office 365, including SharePoint Online. Includes 7 modules, with over 10 hours of video and 17 hands-on-labs.
Have a look at:
Sandboxed Solutions are the development paradigm for SharePoint Online. In this session, you’ll learn about sandboxed solutions including how to develop, debug and deploy solutions. You’ll also learn the breadth of solutions that can be developed in the sandbox and strategies for developing common scenarios that are not enabled in the sandbox.