Moving this thread from Flash Pro FAQs to Flash Pro - General.
Toolkit exports a nominalBounds property for all exported MovieClips. This will give you the first frame untransformed bounds of every symbol. Optionally, you can also enable "multi frame bounds" which will export the bounds for every frame of the MovieClip.
Dynamic, run-time calculation of bounds is unlikely to be added any time soon, due to the extremely high processing requirements to calculate these values, and the level of complexity it introduces to the library.