Apparently only the last 48 F/A-18D's delivered to the Marines have provisioning to be fitted with the recon pallet (though only a smaller number at any point in time have it fitted).
There were a number of earlier attempts to get a recon version of the single and dual seat hornets, however they were never funded.
As much as I don't want to point you to wiki as the source of truth (becuase it isn't), this does have a brief mention of the other two recon types of the Hornet http://en.wikipedia.org/wiki/McDonnell_Douglas_F/A-18_Hornet