This is a job for the Synch robot. In the workflow updating window, it is the rightmost icon on the last line of the robot list. Parameters are quite self explanatory : you select a class, an event that can occur on that class. If needed, you can add some code to further process the event that occured.
In Reply to Ravi Sankaran:
Has anyone developed a workflow that has a sync robot that would monitor foran external event to resume processing?
If you have any pointers to documentation, that would help too.
Bus. Admin guide says that this is possible but no further details.