The bulk of the messages in the journal account on Exchange 2k7 arrive in the appropriate format, message body contains header information while actual message body is attached, but a rather large number of messages are arriving in the journal without this format. The to/from pair of these messages does not show the journal as a recipient, so this isn't a case of the journal receiving these as a recipient. They appear to be journaled using the journal rule, but the format is incorrect.
Looks like I was able to answer my own question. The environment included an Exchange 2003 mail server which was operating as a mail host for certain systems. As a result, mail handled by Exchange 2003 was being journaled to the same location, but absent the proper format. Eliminating the older system resulted in proper journal format for all messages in the journal. It may also be possible to simply configure the 2003 box for envelope journaling in order to rectify this if necessary.