GirlFriday, you'll be happy to know that OmniPlan 2.1 shipped last week, and includes a feature meant to help you balance resource workloads across multiple projects.
More info and the video in
this blog post should help get you started; we're happy to answer any other questions you may have, of course. (Support mail generally gets the quickest response, but we do our best to spend as much time on the forums as we can manage on top of our other duties.)