Taverna does not support looping in the sense that there are no cycles within a workflow graph.
Taverna 2.x supports looping in the sense that it allows you to invoke a service until a particular condition is satisfied. For example, it allows you to poll an asynchronous service until the result is available. To create a loop on a service in Taverna 2.x, select the service in the Workflow Diagram and then click on the Details in the Workflow Explorer. Select Advanced and click Add looping. A pop-up window will then allow you to select an output port and the conditions of your loop. Details of how to configure looping are in the looping section of the user manual.