Not a great deal to go on here, What type of event log are you after?
I'm assuming that you want to track events in a workflow going by your reference to tasks, so if that's the case, you could add a multiline text field to the source item and just use that as an event repository.
I've used this method before along with versioning to add a timestamp automatically, however this was from a code based workflow so if you're using SPD not sure how this would behave, but it's feasible.
Paul.
www.myfatblog.co.uktwitter.com/@cimaresMCTS: WSS 3.0
MCTS: MOSS 2007