The System.Workflow.Runtime.WorkflowRuntime WorkflowDefinitionCacheCapacity property (the number of workflow definitions saved by the workflow runtime) has been added to the .Net 3.5 Framework by a QFE.

This property has already been added in .Net Framework 4.0, and is documented here:

http://msdn.microsoft.com/en-us/library/system.workflow.runtime.configuration.workflowruntimesection.workflowdefinitioncachecapacity.aspx