Found the problem...
I had this squeak issue posted as a thread in the DIY-MAINTENANCE area as well. E90Fleet responded with the problem/fix (much appreciated).
Turns out I did not have the parking brake fully engaged. To test, I disengaged the parking brake and bounced on the door sill... NO SQUEAK! Then I gave the parking brake a good yank, bounced on the sill, no squeak.
Thanks all! I really appreciate that I don't have to waste time going to the dealer (just to be told I need to pull hard on the brake lever).
Live to drive...
E39 2000 528i*
E32 1994 740il*