I have an already existing application in Silverlight 4.0 with WCF services. Now what I want is a workflow to be integrated along with the above application. The workflow will be a simple approval workflow i.e sequential workflow. Can any one suggest some links or a solution?
A workflow is simply applying a state to your application's data.
And the type of workflow you described (simple, sequential) is a really good fit with the async model that WCF requires.
When a user submits a request for a resource, they have to wait until their supervisor (and so on up the chain) approves the request. In the meantime, the user is free to go about other work or simply wait for the resource request to be approved. Polling or notification mechanisms of some sort are required for the user and the approvers. The user needs notification for when the resource is available, and the approvers need notification that they have work (grant | deny approval) to do.
One possibility for those mechanisms is to add a polling routine to your View-Models. Another option would be to look at push type notifications. However, there's nothing within Silverlight or WCF that makes creating a workflow inherently more difficult than with any other framework.