Developing solutions for SharePoint Online requires they be built using Sandboxed Solutions. Sandboxed solutions are developed against a scoped version of the Microsoft.SharePoint API specific to the capabilities of the sandbox.
While Visual Studio 2010 provides IntelliSense support for the sandbox, compilation support would be even better (so you can catch issues with code copy/pasted from other projects or online samples, etc.).
Enter the Visual Studio 2010 SharePoint Power Tools. These tools provide sandboxed solution compilation support and a Visual Web Part (Sandboxed) project item template so you can create Visual Web Parts that will run in the Sandbox.
Download and install the tools today. They are a huge time saver for SharePoint Online and sandboxed solutions development.