In the Apr, 2001 Burton hearings hearings on autism, the FDA rep {Dr. Midthun) said that thimerosol was (voluntarily) being taken out of newly manufactured vaccines, but that existing stocks of vaccines on manufacturers' and pharmacy shelves WERE NOT being recalled.
She offered no details as to how the FDA was going to monitor this voluntary effort by the drug companies.
Given the cozy relationship between the FDA and drug companies which has been discussed on this board many times, you can take all that for what it's worth.
If there are any further questions regarding this, I can post the entire exchange betwen Mr. Burton and Dr. Midthun. It's quite informative, but very lengthy.