As of May 17, 2008, 8.7% of the journals indexed in MEDLINE / PubMed are full participants in PubMedCentral. Of these, 200 are traditional, non-open-access publishers. Good news though this is, this shows that there is much more work to do!
Thanks to Michael Rogawski on the SPARC Open Access Forum. According to Rogawski, there are 5,194 (mostly) biomedical and life sciences journals indexed in MEDLINE/PubMed, and 452 journals fully participating in PMC.
Full participation in PMC is essential for medical journals, not only for access, but for preservation, too.
The approximate number of journals in PubMed (5,200) can be found on the National Library of Medicine's fact sheet, What is the difference between Medline and PubMed?, accessible from the Medline site. (I can't find a link from PubMed, though).
The exact number of journals indexed in Index Medicus / PubMed as of March 2008 according to the NLM website is 5,271.
It will be interesting to review this data in the future as part of the quarterly Dramatic Growth of Open Access Series.