Microsoft have provided clarity around the use of Custom Workflow activities in CRM 2011, you need to be careful if you go the CRM online route that a plug-in extension solution will meet your needs, Workflow extensions are NOT supported.
Here is the Microsoft article release:
We have received multiple questions and feedback regarding the capability of adding custom workflow activities in CRM 2011 Online and I would like to provide some responses as well as offer some guidance on how to insert custom logic to workflows in CRM 2011 Online.
Due to a technical limitation we are unable to support custom workflow activities in CRM 2011 Online. Custom workflow activities is a widely used feature and having direct support for it in Online is one of our top priorities, we are considering enabling this support in a future release.