Source for PKCS#11 Header Files
The PKCS#11 standard has transitioned from RSA to the OASIS group:
https://www.oasis-open.org/committees/tc_home.php?wg_abbrev=pkcs11

I am not sure why RSA/EMC's site doesn't mention this.

I believe that v2.30 was close to finalization, but was never formally released.  I am not sure what happened but v2.40 is now the current release candidate.  

v2.20 is the current revision and header files are accessible by following the link that Mr. Stone provided.

CryptSoft (Tim Hudson is a PKCS#11 leader) has a very good resource page:
http://www.cryptsoft.com/pkcs11doc/