Tom's posting made me think of another reason for rasterization, as layer transparency is just one way to get your map to rasterize.
Tom and Charlie made me think of another instance when things rasterize. I have had rasterization occur when exporting a data that has been grouped into differerent subgroups in the TOC.
For example, when I export something from ESRI's StreetMapUSA.lyr or USA Thematic Maps.lyr that is in a sub-sub group it rasterizes on export to AI, but if I drag the data layer out of the subgroup and into a top level in the TOC is exports as vector.
I assume it has something to do with SDC - but have found very little info about this (new) intriguing (proprietary) data format.