PubMed Central underwent a major redesign, released yesterday. In an e-mail
I got from a staffer there:

From here on pmc will have persistent *journal IDs* and *article IDs*.
These two IDs are used to
construct the URLs to a journal toc or archive page and to an article
abstract or full-text page respectively, as in:

Also it should be noted that there a lot more BMC (BioMed Central) titles,
and of course, in the PMC redesign, their urls changed:

For those libraries maintaining a e journals list, there are some
corrections on individual title urls to be made!

Ramune Kubilius
Collection Development / Special Projects Librarian
Galter Health Sciences Library
Northwestern University