I came up with a way to create accurate DOF effects that is accurate even through transparent objects using Strata and Photoshop a while ago. Now that we've been talking about all the v5 features I'd though I'd share this technique. There are a few steps involved so when v5 arrives we'll all be happy. But even after - if you have many transparent objects this way will be accurate.
See this image for example... http://www.stratacafe.com/gallery/jeffry_gugick/76960218200622243.jpg