Ahhh, interesting... Sounds like you did your homework with the SP logging. The important part of that test was to learn that you are getting parameters, and you are expecting data (not that it's proven that it's getting back to the sub when in the main).
You sound plenty savvy, but just to make sure you haven't overlooked the obvious:
-Make sure that the save data with report is turned off.
-There should not be any suppression being used in the main report section which houses the sub.
-The sub should not have any suppression formula in the details section.
-The sub WILL have a record selection criteria (I doubt that this is a problem, but thus far it's been a well kept secret

)
This is starting to sound a bit bug-like, some tests seem appropriate.
Save the subreport to another name, create a new blank report, and place this subreport into the report footer.
Hardcode the parameter values in the record selection criteria.
Do you see the Subreport?
If so, you either have corruption somewhere, or some formula which is preventing the subreport from showing details.
Let's hear the results.
-k
kai@informeddatadecisions.com