I found Custom Air Specialists through Groupon (under the name "It's Duct Time").
Two technicians came to my house, and explained what they were going to do. They removed all the larger room ducts, and ran a vacuum into each one. They took two hours to vacuum the ducts in my house, which included a cleaning of the two air handlers in the attic which were caked with dirt but did not show any signs of mold or other contamination (note: the handlers were additional cost, but something that the lead tech explained only needed to be done every 4-5 years).
The technician (Teto) seemed very competent and knowledgeable. He explained how the two systems in my house worked and answered several questions for me. He also told me he checked around the attic crawlspace and did not see signs of dirt which indicated the attic is reasonably well sealed from the outside.
He did try to sell me on electrostatic filters to replace my fiberglass ones, and I'm doing some internet research on those now. At first blush it looks like his advice is sound.
** UPDATED ** The company called me as follow-up to make sure I was happy with their work. I'm impressed with their customer service and recommend them to others. Also I have completed my research, and decided to accept their recommendation and install the electrostatic (permanent) filters in my house.