Yesterday the updated document set on Microsoft Online Dedicated Service was published. The document set can be downloaded from Microsoft Download Center. link: Microsoft Online Dedicated Service Descriptions
From the perspective of SharePoint developers, one document would be of particular interest: Microsoft SharePoint Online Custom Solutions Support (SharePoint Online Custom Code Support v9-1.doc). As in on-premise SharePoint environment, there are 3 types of customizations available in SharePoint Online Dedicated service:
Microsoft SharePoint Online Custom Solutions Support document provides a detailed description of support for custom solution development i.e. .NET code components (3rd option above) and dependent files.
To ensure best practices are followed and there is compatibility with future service packs and product releases, the document outlines a solution development and deployment process. Microsoft will review proposed customizations, determine whether an out-of-the-box or existing solution can accomplish the same goals, and then guide the process to ensure the customization is deployable, supportable and sustainable by the Microsoft support organization.