This article http://support.microsoft.com/kb/968755 details how to clear up the AsyncOperationBase and WorkflowLogBase tables
In CRM 4.0 workflow definitions are stored in the workflowbase table. This table contains a ‘header’ row for each workflow and a row for each step of the workflow. Each row in the table has a workflowid and parentworkflowid column. The parentworkflowid column is used to tie the steps in a particular workflow together. In the ‘header’ row for a workflow, the parentworkflowid value is null.
Each time a workflow is run, a single row is created in the asyncoperationbase table. The workflowactivationid column in asyncoperationbase identifies the specific step of the workflow that started the workflow. Each row in the asyncoperationbase table has a asyncoperationid column (this acts as the primary key).
The detailed history of the workflow history is stored in the workflowlogbase table with one row per workflow step. The asyncoperationid column in workflowlogbase is used to link back to the asyncoperationbase table.