This idea has been merged with Layer consolidation should not be applied when exporting transparent layers to PDF.
If a vector layer with a transparency set on the Display tab of its properties is above other layers then when the map is exported to PDF all the layers underneath are fused together.
http://resources.arcgis.com/content/kbase?fa=articleShow&d=30882 confirms the issue when it says "Layers that cause rasterization, such as transparent layers or layers that use a picture fill symbology, consolidate all the layers below them into a single 'Image' layer."
I've tested at ArcGIS 10 and the problem is still present there.
It seems to be an issue for a few other people:
An FAQ which is possibly related suggests this may come in a later release ...
Could that "later release" be ArcGIS 10.1, please?